patch 8.2.3987: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/mbyte.c b/src/mbyte.c
index e9febcb..2f4edd5 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -513,7 +513,7 @@
 	else if (GetLastError() == ERROR_INVALID_PARAMETER)
 	{
 codepage_invalid:
-	    return N_("E543: Not a valid codepage");
+	    return N_(e_not_valid_codepage);
 	}
     }
 #endif