patch 8.2.3830: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/digraph.c b/src/digraph.c
index adabc9b..a9fcba5 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -2043,7 +2043,7 @@
     }
     if (char1 == ESC || char2 == ESC)
     {
-	emsg(_("E104: Escape not allowed in digraph"));
+	emsg(_(e_escape_not_allowed_in_digraph));
 	return FALSE;
     }
     return TRUE;
@@ -2622,7 +2622,7 @@
 
     if (!getline_equal(eap->getline, eap->cookie, getsourceline))
     {
-	emsg(_("E105: Using :loadkeymap not in a sourced file"));
+	emsg(_(e_using_loadkeymap_not_in_sourced_file));
 	return;
     }