AssertThrows: handle unexpected exceptions

[blueyed]

AssertThrows might pass with something unexpected.

I think this can be improved/detected in Vader by looking at v:throwpoint: if it is function vader#assert#throws, line 5 then it means it's an undefined variable etc.

if v:throwpoint ==# 'function vader#assert#throws, line 5'
  throw 'Vader: unexpected exception: ' . v:exception
endif

Not sure if there is a use case for anything to throw directly from exp?!

Originally posted by @blueyed in #162 (comment)