Teach git apply to provide more descriptive error messages

[Denton-L]

Currently, when running git apply or git am, the error message is infuriatingly bad. It reports something like error: corrupt patch at line 62 and just dies.

We could help users out by providing more information on why it dies. Unexpected character? Too many lines? Too few lines? Something else? Anyway, let's try to avoid dying without giving users a bit more to work with.