patch 9.0.0740: prop_add_list() gives multiple errors for invalid argument

Problem:    prop_add_list() gives multiple errors for invalid argument.
Solution:   Only give one error message.
diff --git a/src/globals.h b/src/globals.h
index 0228825..9d70da6 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -219,8 +219,8 @@
 
 EXTERN int	did_endif INIT(= FALSE);    // just had ":endif"
 #endif
-EXTERN int	did_emsg;		    // set by emsg() when the message
-					    // is displayed or thrown
+EXTERN int	did_emsg;		    // incremented by emsg() when a
+					    // message is displayed or thrown
 #ifdef FEAT_EVAL
 EXTERN int	did_emsg_silent INIT(= 0);  // incremented by emsg() when
 					    // emsg_silent was set and did_emsg