add a user-friendly message how to create config

[astanin]

Two changes to improve first user experience:

• print instructions how to create config and register the app if the config is missing
• create sync_dir automatically if it doesn't exist

Currently the program just dumps a backtrace in both cases.

Backtrace on the first run of the program could be confusing to many users. Personally I ran it through strace to find out where I should put the config. While the information is already in the README, I believe that a giving more specific suggestions to the user can improve user experience.

Before this patch:

$ ./onedrive 
object.Exception@src/config.d(16): No config file found
----------------
??:? ref config.Config config.Config.__ctor(immutable(char)[][]...) [0x5b8666]
??:? _Dmain [0x5bd11a]
??:? _D2rt6dmain211_d_run_mainUiPPaPUAAaZiZ6runAllMFZ9__lambda1MFZv [0x5d8a42]
??:? void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).tryExec(scope void delegate()) [0x5d8980]
??:? void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).runAll() [0x5d89fe]
??:? void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).tryExec(scope void delegate()) [0x5d8980]
??:? _d_run_main [0x5d88dd]
??:? main [0x5bf915]
??:? __libc_start_main [0x67d7dec4]

After this patch:

$ ./onedrive 
Configuration file is not found.

If this is the first time you run onedrive, you should create a configuration
file in /home/sergey/.config/onedrive/config or /usr/local/etc/onedrive.conf or /etc/onedrive.conf. Run:

mkdir -p ~/.config/onedrive
cat > ~/.config/onedrive/config << EOF
client_id = ""
client_secret = ""
sync_dir = "~/OneDrive"
skip_file = ".*|~*"
skip_dir = ".*"
EOF

Then edit ~/.config/onedrive/config.
To get client_id and client_secret, please register this application at
https://dev.onedrive.com/app-registration.htm
and copy both keys from App Settings.

object.Exception@src/config.d(38): No config file found
----------------
??:? ref config.Config config.Config.__ctor(immutable(char)[][]...) [0x5b9285]
??:? _Dmain [0x5bdd4a]
??:? _D2rt6dmain211_d_run_mainUiPPaPUAAaZiZ6runAllMFZ9__lambda1MFZv [0x5d9736]
??:? void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).tryExec(scope void delegate()) [0x5d9674]
??:? void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).runAll() [0x5d96f2]
??:? void rt.dmain2._d_run_main(int, char**, extern (C) int function(char[][])*).tryExec(scope void delegate()) [0x5d9674]
??:? _d_run_main [0x5d95d1]
??:? main [0x5c0545]
??:? __libc_start_main [0x3d546ec4]

[Scimmia22]

This is pretty much moot now, isn't it?

[skilion]

Now the config file is optional