Put full filepaths in error messages when not using the default message format.
Put full filepaths in error messages when not using the default message format.

When the default "antlr" message format is used, ErrorManager deliberately
only includes the filename in any error message, not the full path, to
keep the messages short.  However, if the error message is intended to be
parsed by an IDE, this is no good.

Address this by printing the full path when using alternate message formats.
There are two formats available today -- "gnu" and "vs2005".  I think it's
reasonable to assume that people would use these for IDE compatibility, and
so using the full path for these is appropriate.  Also, they didn't work
until very recently, so this isn't breaking any existing behavior.

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