Issue 11542 & 12809 - Improve blockExit calculation on try-catch-finally statement

[Issue 11542](https://issues.dlang.org/show_bug.cgi?id=11542) - scope(failure) messes up nothrow checking
[Issue 12809](https://issues.dlang.org/show_bug.cgi?id=12809) - More strict nothrow check for try-finally statement