build: Add missing [[noreturn]] to handleRunawayException(...). Enable -Wsuggest-attribute=noreturn if available.

[practicalswift]

• Add missing [[noreturn]] to handleRunawayException(…)
• Enable -Wsuggest-attribute=noreturn if available

[fanquake]

macOS travis:

  CXXLD    test/test_bitcoin_fuzzy
  CXXLD    bitcoind
qt/bitcoin.cpp:527:3: error: function declared '[[noreturn]]' after its first declaration
[[noreturn]] void BitcoinApplication::handleRunawayException(const QString &message)
  ^
qt/bitcoin.cpp:215:10: note: declaration missing '[[noreturn]]' attribute is here
    void handleRunawayException(const QString &message);
         ^
1 error generated.
Makefile:8653: recipe for target 'qt/qt_bitcoin_qt-bitcoin.o' failed

[DrahtBot]

Note to reviewers: This pull request conflicts with the following ones:

• Add BitcoinApplication & RPCConsole tests #11625 (WIP: Add BitcoinApplication & RPCConsole tests by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.