fail fast when the baserom.gbc file doesn't exist

One day this will not be required. But until then, waiting one or more
seconds for the build to fail just to learn that you forgot baserom.gbc
is really silly.

This makes failure almost instantaneous compared to before, which would
take between one and ten seconds depending on system performance.

fixes #167
Merge pull request #169 from kanzure/fail-fast-without-baserom

Fail fast when the baserom.gbc file doesn't exist.