[m-rev.] diff: simplify handling of foreign_export_enums

[Zoltan Somogyi]

Would anyone object if I reworded the error messages
for foreign_enums and foreign_export_enums? Instead of
talking about mappings between Mercury and foreign languages names
"not [being] a bijection", I think they should say something along the
lines of " the mapping is inconsistent", while giving the names of the
Mercury or foreign names that are mapped to more than one name
in the other language.

Zoltan.
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