patch 8.2.3750: error messages are everywhere

Problem:    Error messages are everywhere.
Solution:   Move more error messages to errors.h and adjust the names.
diff --git a/src/mark.c b/src/mark.c
index c708e07..099214e 100644
--- a/src/mark.c
+++ b/src/mark.c
@@ -578,7 +578,7 @@
 {
     if (pos == NULL)
     {
-	emsg(_(e_umark));
+	emsg(_(e_unknown_mark));
 	return FAIL;
     }
     if (pos->lnum <= 0)