[MERGE][#248895] ``bzr ignore`` has better error message when versioned files match ignore pattern

[Stephen J. Turnbull]

Neil Martinsen-Burrell writes:

 > It's a simple change of language, but even the easy bugs need fixing.

No, it's a very complex change of language, needing to be done in at
least four places (are you *sure* you got them all? ;-)  Shouldn't
this be refactored to use a common warning object?