Teach git apply to provide more descriptive error messages

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.