patch 8.2.4010: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move more error messages to errors.h.
diff --git a/src/usercmd.c b/src/usercmd.c
index 72c011b..d4ed0a4 100644
--- a/src/usercmd.c
+++ b/src/usercmd.c
@@ -1101,7 +1101,7 @@
     else if ((name_len == 1 && *name == 'X')
 	  || (name_len <= 4
 		  && STRNCMP(name, "Next", name_len > 4 ? 4 : name_len) == 0))
-	emsg(_("E841: Reserved name, cannot be used for user defined command"));
+	emsg(_(e_reserved_name_cannot_be_used_for_user_defined_command));
     else if (compl > 0 && (argt & EX_EXTRA) == 0)
     {
 	// Some plugins rely on silently ignoring the mistake, only make this