Make check_options raise an error instead of exiting.
The SCMClient.check_options method as written would print an error message to

stderr and called sys.exit(). This is kind of a crummy design, which meant that

running our test suite would have some errors printed in the middle of the

suite output.
I've changed this to raise an exception instead, and replicated the print/exit

behavior in the two non-test places that call it.

Ran unit tests.