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.

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.

Instead of exiting with an error, handle these errors internally.
This will enable a few improvements in the future.

Previously the loader would only report the first error. With this
change, all syntax and type errors will be reported.