This refactoring forces callers to think about how to handle projects
that do not exist explicitly. As a general guidance, API handlers will
want to throw a BadRequestException and components in the inner works of
Gerrit where content should have been pre-validated should throw an
IllegalStateException.
But overall, it's a case-by-case decision.
In a follow-up commit, we will remove #checkedGet and move callers to
get instead.
Change-Id: I95bbc22ad5f3279e6f40f1d05c8d7235d601e32d
a3b587c0ce