Exception messages weren't being bubbled up to the API because the
base exception class wasn't printing correctly. This adds a string
and representation function to ensure they print properly and show
up correctly when debugging interactively.
Cleaned up the `message` attr on the exception classes. It looks
like they started out all without a period, but started adding them
later. Changed classes that were setting error `details` == `message`
to use the default details provided in RESTError.
Change-Id: I1ce256585c9a574e1d1f857c7dc4c417a56b913b
108599f3f0