patch 8.2.3190: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move error messages to errors.h and give them a clear name.
diff --git a/src/map.c b/src/map.c
index 9f80ba8..09339a2 100644
--- a/src/map.c
+++ b/src/map.c
@@ -2723,7 +2723,8 @@
     {
 	case 1: emsg(_(e_invarg));
 		break;
-	case 2: emsg((isabbrev ? _(e_no_such_abbreviation) : _(e_nomap)));
+	case 2: emsg((isabbrev ? _(e_no_such_abbreviation)
+						      : _(e_no_such_mapping)));
 		break;
     }
 }