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.