patch 8.0.0702: an error in a timer can make Vim unusable

Problem:    An error in a timer can make Vim unusable.
Solution:   Don't set the error flag or exception from a timer.  Stop a timer
            if it causes an error 3 out of 3 times.  Discard an exception
            caused inside a timer.

src/structs.h

@@ -3243,6 +3243,7 @@ struct timer_S
     long	tr_interval;	    /* msec */
     char_u	*tr_callback;	    /* allocated */
     partial_T	*tr_partial;
+    int		tr_emsg_count;
 #endif
 };