Fix the C++ exception struct in nimbase.h.

The struct NimException still referred to the old exception type
E_Base, which has since been renamed to Exception. This made the
C++ backend fail on any code that used exceptions.
implemented 'guard' annotation
Merge branch 'bigbreak' of https://github.com/Araq/Nimrod into bigbreak
made some tests green
Merge pull request #1536 from rbehrends/fix-c++-exceptions

Fix the C++ exception struct in nimbase.h.
Merge branch 'bigbreak' of https://github.com/Araq/Nimrod into bigbreak
made tests green