Validate review hashes when loading a review.
Validate review hashes when loading a review.

This change improves the loading of reviews in two ways:

1. Do not return a nil error from `getSummaryFromNotes` if the returned
   summary is nil, as that breaks assumptions made by callers that one
   of the two return values must be non-nil.
2. Proactively check that the specified hash exists and points to a commit
   object before trying to load a review attached to that commit.

The combination of these two changes fixes #73

The first change prevents the tool from panicing on a bad input, while
the second change makes the error message presented to the user easier
to understand and fix.

google/git-appraise