cmd/cue: fix error on cue mod rename with a bad module

We weren't printing the error in this case.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: Ib8da7ac07c52eeb61c4a983e855919e2b927918f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1216869
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

cmd/cue/cmd/modrename.go

@@ -60,7 +60,7 @@ type modRenamer struct {
 func runModRename(cmd *Command, args []string) error {
 	modFilePath, mf, _, err := readModuleFile()
 	if err != nil {
-		return nil
+		return err
 	}
 	if mf.Module == args[0] {
 		// Nothing to do

cmd/cue/cmd/testdata/script/modrename_badmodule.txtar

@@ -1,8 +1,10 @@
 # Test that we see an error when the module cannot be loaded
 
-# TODO this should be an error
-exec cue mod rename other.org
+! exec cue mod rename other.org
+cmp stderr want-stderr
 
+-- want-stderr --
+no language version declared in module.cue
 -- cue.mod/module.cue --
 // Note: no language version
 module: "main.org"