Fix #597; ProofGeneral cannot step over Fail correctly

The bug happened when `Set Ltac Backtrace` is enabled, which has
become the default since a few versions. The fix is to prefix the
coq-error-message variable with a negative regexp for "message:\n".

This PR should be accepted if it does not slow performances too much.

coq/coq-syntax.el

@@ -1282,7 +1282,7 @@ different."
 (defvar coq-symbols-regexp (regexp-opt coq-symbols))
 
 ;; ----- regular expressions
-(defvar coq-error-regexp "^\\(In nested Ltac call\\|Error:\\|Discarding pattern\\|Syntax error:\\|System Error:\\|User Error:\\|User error:\\|Anomaly[:.]\\|Toplevel input[,]\\)"
+(defvar coq-error-regexp "\\(?:[^:]\\|[^e]:\\|[^g]e:\\|[^a]ge:\\|[^s]age:\\|[^s]sage:\\|[^e]ssage:\\|[^m]essage:\\)\n\\(In nested Ltac call\\|Error:\\|Discarding pattern\\|Syntax error:\\|System Error:\\|User Error:\\|User error:\\|Anomaly[:.]\\|Toplevel input[,]\\)"
   "A regexp indicating that the Coq process has identified an error.")
 
 ;; april2017: coq-8.7 removes special chars definitely and puts