gdb: merge error handling from different expression parsers

gdb: merge error handling from different expression parsers

Many (all?) of the expression parsers implement yyerror to handle
parser errors, and all of these functions are basically identical.

This commit adds a new parser_state::parse_error() function, which
implements the common error handling code, this function can then be
called from all the different yyerror functions.

The benefit of this is that (in a future commit) I can improve the
error output, and all the expression parsers will benefit.

This commit is pure refactoring though, and so, there should be no
user visible changes after this commit.

Approved-By: John Baldwin <jhb@FreeBSD.org>