patch 8.2.3967: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more errors to errors.h.
diff --git a/src/optionstr.c b/src/optionstr.c
index 85405ed..49d23b0 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -380,7 +380,7 @@
 	idx = findoption(name);
 	if (idx < 0)	// not found (should not happen)
 	{
-	    semsg(_(e_intern2), "set_string_option_direct()");
+	    semsg(_(e_internal_error_str), "set_string_option_direct()");
 	    siemsg(_("For option %s"), name);
 	    return;
 	}