svnmeta: improve error message in get_source_rev() Letting a KeyError propagate when the supplied revision has no conversion record on file isn't terribly helpful. At least not when this KeyError merely states that the key missing is 'convert_revision'. It's much better to preemptively check for it, detect that it's missing, and then raise an exception with a descriptive message.

import imp, os, sys

sys.path.insert(0, os.path.dirname(os.path.dirname(__file__)))