provide nicer feedback when an unknown node id is passed to a command
Previously, an unknown node id would lead to the following error:
abort: 00changelog.i@343445453433: no node!
All other unknown revision would instead display as:
abort: unknown revision '343445453'!
The former error message has been suppressed in favor of the latter.
tests: check for bzr support by importing bzrlib
This is better than starting bzr --version, because it works correctly when
running tests with a non-default Python (version).