[merge][#251352] better messages for illegal 'log -m' arguments

[Martin Pool]

Based on Anne Mohsen's patch in
<https://bugs.edge.launchpad.net/bzr/+bug/251352> this gives a nicer
message when the user gives an illegal regexp.  (Thanks, Anne.)  It
also separates out a function re_compile_checked that we should use
whenever compiling a user-supplied regexp because the python default
error is not very helpful.

I haven't added this into the ignore code (yet) because it's not
directly applicable and that code already makes some effort not to
compile them one by one for the sake of speed.

-- 
Martin <http://launchpad.net/~mbp/>
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