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/optionstr.c b/src/optionstr.c
index f4a8cb8..243d10f 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -1026,8 +1026,8 @@
 		if (convert_setup(&input_conv, p_tenc, p_enc) == FAIL
 			|| convert_setup(&output_conv, p_enc, p_tenc) == FAIL)
 		{
-		    semsg(_("E950: Cannot convert between %s and %s"),
-			    p_tenc, p_enc);
+		    semsg(_(e_cannot_convert_between_str_and_str),
+								p_tenc, p_enc);
 		    errmsg = e_invalid_argument;
 		}
 	    }