[testsuite] don't ignore errors for dg-error and friends

[Janis Johnson]

This patch prevents errors in dg-error, dg-warning, and dg-message from
being silently ignored by GCC's overrides for those procedures.  It's
been tested on i686-pc-linux-gnu for the languages that use the overrides
(c,c++,fortran,objc,obj-c++).  I scanned through target-specific tests
and didn't see anything problematic, but it's possible I missed something
and errors will be reported.  Checked in on mainline, and I plan to add
it to 4.7 soon.

Janis
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