move validation of pointer Button into conf_mousebind so we check

validity during the parse phase and not bother adding it to the queue,
instead of each time a client needs to grab (when it's too late);
furthermore, make this a config error, stop parsing and load the
default config.

parse.y

@@ -171,7 +171,12 @@ main		: FONTNAME STRING		{
 			conf->gap.right = $5;
 		}
 		| MOUSEBIND STRING string	{
-			conf_mousebind(conf, $2, $3);
+			if (!conf_mousebind(conf, $2, $3)) {
+				yyerror("invalid mousebind: %s %s", $2, $3);
+				free($2);
+				free($3);
+				YYERROR;
+			}
 			free($2);
 			free($3);
 		}