patch 8.2.4012: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move the last error messages to errors.h.
diff --git a/src/diff.c b/src/diff.c
index 5a98196..4136d99 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -1139,7 +1139,7 @@
 		&diffio->dio_new.din_mmfile,
 		&param, &emit_cfg, &emit_cb) < 0)
     {
-	emsg(_("E960: Problem creating the internal diff"));
+	emsg(_(e_problem_creating_internal_diff));
 	return FAIL;
     }
     return OK;
@@ -1733,7 +1733,7 @@
 	    }
 	    else
 	    {
-		emsg(_("E959: Invalid diff format."));
+		emsg(_(e_invalid_diff_format));
 		break;
 	    }
 	}