cmd/cue: improve error message when modules experiment not enabled

When `cue mod tidy` or `cue mod publish` are run, which both require
modules to be enabled, the current error seems to indicate that
there is no registry configured, even though there's a default registry
fallback, so that shouldn't be necessary.

Change the error message to suggest setting CUE_EXPERIMENT=modules
instead.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I57348e595154ab99a0a6ada09fa80584a62958dc
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1177130
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

cmd/cue/cmd/modpublish.go

@@ -56,7 +56,7 @@ func runModUpload(cmd *Command, args []string) error {
 		return err
 	}
 	if resolver == nil {
-		return fmt.Errorf("no registry configured to publish to")
+		return fmt.Errorf("modules experiment not enabled (enable with CUE_EXPERIMENT=modules)")
 	}
 	modRoot, err := findModuleRoot()
 	if err != nil {

cmd/cue/cmd/modtidy.go

@@ -61,7 +61,7 @@ func runModTidy(cmd *Command, args []string) error {
 		return err
 	}
 	if reg == nil {
-		return fmt.Errorf("no module registry configured")
+		return fmt.Errorf("modules experiment not enabled (enable with CUE_EXPERIMENT=modules)")
 	}
 	ctx := context.Background()
 	modRoot, err := findModuleRoot()

cmd/cue/cmd/testdata/script/modpublish_registry_not_enabled.txtar

@@ -0,0 +1,4 @@
+! exec cue mod publish v1.0.0
+cmp stderr want-stderr
+-- want-stderr --
+modules experiment not enabled (enable with CUE_EXPERIMENT=modules)

cmd/cue/cmd/testdata/script/modtidy_registry_not_enabled.txtar

@@ -0,0 +1,4 @@
+! exec cue mod tidy
+cmp stderr want-stderr
+-- want-stderr --
+modules experiment not enabled (enable with CUE_EXPERIMENT=modules)