bzr: Improve error messages a bit, and assume updates were by bzr.

• 'Internal' exceptions are only for things that are probably

kdesrc-build errors. They include an ugly backtrace. Normal errors
outside of kdesrc-build purview (including bzr's own errors) should just
be 'runtime' exceptions.

• We don't try to introspect from bzr how many changes (commits, files,

etc.) were made in a bzr pull. Mostly because I don't know best way to
do it. In this situation the most conservative thing to do is assume an
update was made, because if this code reports back that there were no
changes, kdesrc-build may not even try to rebuild the module.