Fixes #251: segfault (stopClient) and showing error (not active window)

[rNoz]

First, thanks for the application :)

This PR fixes these two errors, reported here #251
It also provides some improvements related to error reporting.

1. Segfault.

Debugged and found.

(gdb) bt
#0  0x0000fffff6450cb4 in QObject::deleteLater() () from /lib64/libQt6Core.so.6
#1  0x000000000056f234 [PAC] in MainWindow::stopClient (this=0xffffffffdb90) at /home/user/spotify-qt/src/mainwindow.cpp:597
#2  0x000000000056f128 in MainWindow::startClient (this=0xffffffffdb90) at /home/user/spotify-qt/src/mainwindow.cpp:584
#3  0x00000000004ff3e8 in SettingsPage::Spotify::restartClient (this=0x126c380) at /home/user/spotify-qt/src/settingspage/spotify.cpp:173
#4  0x00000000005048f8 in QtPrivate::FunctorCall<QtPrivate::IndexesList<0>, QtPrivate::List<bool>, void, void (SettingsPage::Spotify::*)(bool)>::call (
    f=(void (SettingsPage::Spotify::*)(SettingsPage::Spotify * const, bool)) 0x4ff2cc <SettingsPage::Spotify::restartClient(bool)>, o=0x126c380, arg=0xffffffffcd18)
    at /usr/include/qt6/QtCore/qobjectdefs_impl.h:145
#5  0x000000000050422c in QtPrivate::FunctionPointer<void (SettingsPage::Spotify::*)(bool)>::call<QtPrivate::List<bool>, void> (
    f=(void (SettingsPage::Spotify::*)(SettingsPage::Spotify * const, bool)) 0x4ff2cc <SettingsPage::Spotify::restartClient(bool)>, o=0x126c380, arg=0xffffffffcd18)
    at /usr/include/qt6/QtCore/qobjectdefs_impl.h:182
#6  0x0000000000503610 in QtPrivate::QCallableObject<void (SettingsPage::Spotify::*)(bool), QtPrivate::List<bool>, void>::impl (which=1, this_=0x12638a0, r=0x126c380, a=0xffffffffcd18, ret=0x0)
    at /usr/include/qt6/QtCore/qobjectdefs_impl.h:553
#7  0x0000fffff64623c0 in void doActivate<false>(QObject*, int, void**) () from /lib64/libQt6Core.so.6
#8  0x0000fffff7a612b4 [PAC] in QAbstractButton::clicked(bool) () from /lib64/libQt6Widgets.so.6
#9  0x0000fffff7a6159c [PAC] in QAbstractButtonPrivate::emitClicked() () from /lib64/libQt6Widgets.so.6
#10 0x0000fffff7a62e2c [PAC] in QAbstractButtonPrivate::click() () from /lib64/libQt6Widgets.so.6
#11 0x0000fffff7a63028 [PAC] in QAbstractButton::mouseReleaseEvent(QMouseEvent*) () from /lib64/libQt6Widgets.so.6
#12 0x0000fffff799eb2c [PAC] in QWidget::event(QEvent*) () from /lib64/libQt6Widgets.so.6
#13 0x0000fffff793e9f8 [PAC] in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib64/libQt6Widgets.so.6
#14 0x0000fffff7948270 [PAC] in QApplication::notify(QObject*, QEvent*) () from /lib64/libQt6Widgets.so.6
#15 0x0000fffff63f20b8 [PAC] in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /lib64/libQt6Core.so.6
#16 0x0000fffff7947558 [PAC] in QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) () from /lib64/libQt6Widgets.so.6
#17 0x0000fffff79b25e0 [PAC] in QWidgetWindow::handleMouseEvent(QMouseEvent*) () from /lib64/libQt6Widgets.so.6
#18 0x0000fffff79b4f3c [PAC] in QWidgetWindow::event(QEvent*) () from /lib64/libQt6Widgets.so.6
#19 0x0000fffff793e9f8 [PAC] in QApplicationPrivate::notify_helper(QObject*, QEvent*) () from /lib64/libQt6Widgets.so.6
#20 0x0000fffff63f20b8 [PAC] in QCoreApplication::notifyInternal2(QObject*, QEvent*) () from /lib64/libQt6Core.so.6
#21 0x0000fffff6e83b58 [PAC] in QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) () from /lib64/libQt6Gui.so.6
#22 0x0000fffff6ef2544 [PAC] in QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib64/libQt6Gui.so.6
#23 0x0000fffff7383118 [PAC] in userEventSourceDispatch(_GSource*, int (*)(void*), void*) () from /lib64/libQt6Gui.so.6
#24 0x0000fffff58b09b4 [PAC] in g_main_context_dispatch_unlocked.lto_priv () from /lib64/libglib-2.0.so.0
#25 0x0000fffff59157a4 [PAC] in g_main_context_iterate_unlocked.isra () from /lib64/libglib-2.0.so.0
#26 0x0000fffff58b2084 [PAC] in g_main_context_iteration () from /lib64/libglib-2.0.so.0
#27 0x0000fffff66f1818 [PAC] in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib64/libQt6Core.so.6
#28 0x0000fffff63ff9d4 [PAC] in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /lib64/libQt6Core.so.6
#29 0x0000fffff63faf7c [PAC] in QCoreApplication::exec() () from /lib64/libQt6Core.so.6
#30 0x000000000056be04 [PAC] in main (argc=1, argv=0xffffffffe4d8) at /home/user/spotify-qt/src/main.cpp:107

2. Error messages when in settings panel.

I don't know what is the use case to only show the error messages when the current window (main one) is the active one, as those messages are coming from the settings window. But I have found a problematic behavior with it, reported in the issue. I have spent a bunch of time trying to guess what was the error here (not considering that it was a configuration error, since first it segfaulted due to using a null ptr). If I saw the error "no username", I would have just checked configuration and changed it. So, definitely, there is value in showing errors independently of the active window.

Also, I don't get why we are not showing an error with Bad credentials if we are still not using the KEYCHAIN. So, I have modified it to be able to see this message:

Furthermore, as it can be seen, I propose changing the beginning of that message (from "Failed to start Spotify client:" to "Spotify client error:"), as it will make sense in all scenarios. For instance, when spotify client crashes (e.g. myself killing the librespot process):

(You can see how I propose changing also the error messages from "Error 1" to "Process crashed").

Furthermore, since we can stop the client on purpose, and to avoid modifying the settings panel logic, I have updated that message to be "Process stopped or crashed", as can be both cases. I am reusing the mainContent variable to detect if we are closing the app to avoid showing error messages (established in event->accept() on closeEvent)

Then, clicking in "Stop client":

But not errors are shown in a dialog. The same when we close the application window.