Handle exceptions in the non-main thread properly

[The-Compiler]

If an exception happens in the non-main thread, we currently try to invoke the exception dialog from a non-main thread, which causes a lot of further trouble (accessing sqlite and doing GUI-stuff from the wrong thread).

Instead, we should probably check what the current thread is, and do something more sane. This might also make @utils.prevent_exceptions unnecessary, at least in some scenarios.