Output a better error message when running rbt patch --print

Review Request #9490 — Created Jan. 16, 2018 and discarded — Latest diff uploaded

Information

RBTools
release-0.7.x

Reviewers

When running rbt patch --print outside of a repository directory and
without providing the --server parameter or having a ~/.reviewboardrc
file containing REVIEWBOARD_URL, the command would output that the
current directory does not contain a supported repository.

The command is expected to work outside these repository directories. By
improving the error handling in rbtools/clients/__init__.py and in
rbtools/commands/patch.py we can output a more precise error message.

Tested rbt patch --print both inside and outside of directories with
.reviewboardrc files, as well as using --server. Manually verified
other functions calling initialize_scm_tool will continue to function
as is and bubble up the new exceptions being raised.