internal/mod/module: improve error message

We can distinguish between "not well formed" and "is not canonical".

Following suggestion here:
https://review.gerrithub.io/c/cue-lang/cue/+/1168707/comment/031030c7_0f2c5409

For #2330.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I1dab05fc03cfcb856b54d1f5757372808a685049
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1168802
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: rogpeppe

internal/mod/module/module.go

@@ -108,6 +108,9 @@ func MustNewVersion(path string, vers string) Version {
 // if the version isn't empty; if the version is empty, it's an error.
 func NewVersion(path string, vers string) (Version, error) {
 	if vers != "" && vers != "none" {
+		if !semver.IsValid(vers) {
+			return Version{}, fmt.Errorf("version %q (of module %q) is not well formed", vers, path)
+		}
 		if semver.Canonical(vers) != vers {
 			return Version{}, fmt.Errorf("version %q (of module %q) is not canonical", vers, path)
 		}

internal/mod/module/module_test.go

@@ -83,6 +83,10 @@ var newVersionTests = []struct {
 	path:      "foo.com/bar",
 	vers:      "",
 	wantError: `path "foo.com/bar" has no major version`,
+}, {
+	path:      "x.com",
+	vers:      "bad",
+	wantError: `version "bad" \(of module "x.com"\) is not well formed`,
 }}
 
 func TestNewVersion(t *testing.T) {