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.

97f2079e3778511944ffb6a662520580650a3993 1.0
09c016174e332eceda015d3f43d96d7e0228acf3 1.0.1
4359ddd73b009fbb356c52ea5d99cba25222ee7a 1.1
8e621dbb82d4363a85317638ad237e2817c56347 1.1.1
093ae2915b452539b44390ee4ea14987484e1eee 1.1.2