[m-rev.] for review: changes to error messages

[Zoltan Somogyi]

I have just about finished a change that stores type, inst and mode
definitions in parse_tree_module_srcs in checked form. This implies
that any errors in those definitions are now diagnosed by the code in
check_type_inst_mode_defns, not the code in add_type.m and add_mode.m,
and indeed I intend for my next change to make the code
in those two modules, which right now still process items individually,
to take checked definitions as input, and just record them in the HLDS.

The two sets of diagnostics (one in check_type_inst_mode_defns.m,
the other in add_{type,mode}.m) were developed mostly independently.
Sometimes they report different info, and usually they use different wording.
I thought I would get your feedback, for each test case involving these
diagnostics, whether

- you prefer the old diagnostic
- you prefer the new diagnostic
- or you would prefer a blend of the two.

So please have a look at the attached file which contains all the
relevant parts of a bootcheck output, and tell me your opinions.

Zoltan.
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