internal/mod/modload: better error message for not-found module

When a module isn't found we should give an error that
reflects that fact.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I9227dad4f241bc696502bda0ed699a59dbb8ec29
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201493
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

Author: rogpeppe

cmd/cue/cmd/testdata/script/modtidy_not_found.txtar

@@ -6,7 +6,7 @@ module: "main.org@v0"
 language: version: "v0.10.0"
 
 -- want-stderr --
-path "other.test" has no major version
+no versions found for module other.test@v0
 -- main.cue --
 package main
 

internal/mod/modload/update.go

@@ -150,6 +150,10 @@ func resolveUpdateVersions(ctx context.Context, reg Registry, rs *modrequirement
 					possibleVersions = append(possibleVersions, v)
 				}
 			}
+			if len(possibleVersions) == 0 {
+				setError(fmt.Errorf("no versions found for module %s", v))
+				return
+			}
 			chosen := latestVersion(possibleVersions)
 			mv, err := module.NewVersion(mpath, chosen)
 			if err != nil {