Markdown: Don't crash when opening too many files

Wait for document to exist
Compilers: Don't crash on save

Don't await an OnlyOnFaulted continuation.
Compilers: Replace async void with DontWait()

Catch errors instead of crashing VS
Project: Fix async method names

All methods that return tasks should end in Async.
Also introduces more DontWait() calls to handle exceptions.
Tests: Add delay for solution settings test

Fixes occasional failures
Merge pull request #559 from SLaks/misc-fixes

Exception handling fixes