Match the error messages in testdata/errors/*.go like LLVM FileCheck,
which includes regular expressions.
I believe this is much more flexible, and LLVM uses it in nearly all of
their tests so it must work quite well for compilers.
This is done at a later time anyway, so doesn't need to be done in the
cgo package. In fact, _not_ doing it there makes it easier to print
correct relative packages.
This is needed for some improvements I'm going to make next.
This commit also refactors error handling slightly to make it more
easily testable, this should hopefully not result in any actual changes
in behavior.
Ayke van Laethem