bzr could hit internal errors for various reasons. A common one that

would impact our customers is an incompatibility between bzr and some

versions of paramiko. We now check for internal errors and translate

them to something more suitable if needed.
The paramiko issue was handled on Review Board 2.5.x previously. This

is partly a backport.

Unit tests pass.
Saw the paramiko-related error, and the related skip test.