Improve error messages for PostCSS etc.

Fixes #9730

common/herrors/errors.go

@@ -59,11 +59,34 @@ func GetGID() uint64 {
 	return n
 }
 
+// IsFeatureNotAvailableError returns true if the given error is or contains a FeatureNotAvailableError.
+func IsFeatureNotAvailableError(err error) bool {
+	return errors.Is(err, &FeatureNotAvailableError{})
+}
+
 // ErrFeatureNotAvailable denotes that a feature is unavailable.
 //
 // We will, at least to begin with, make some Hugo features (SCSS with libsass) optional,
 // and this error is used to signal those situations.
-var ErrFeatureNotAvailable = errors.New("this feature is not available in your current Hugo version, see https://goo.gl/YMrWcn for more information")
+var ErrFeatureNotAvailable = &FeatureNotAvailableError{Cause: errors.New("this feature is not available in your current Hugo version, see https://goo.gl/YMrWcn for more information")}
+
+// FeatureNotAvailableError is an error type used to signal that a feature is not available.
+type FeatureNotAvailableError struct {
+	Cause error
+}
+
+func (e *FeatureNotAvailableError) Unwrap() error {
+	return e.Cause
+}
+
+func (e *FeatureNotAvailableError) Error() string {
+	return e.Cause.Error()
+}
+
+func (e *FeatureNotAvailableError) Is(target error) bool {
+	_, ok := target.(*FeatureNotAvailableError)
+	return ok
+}
 
 // Must panics if err != nil.
 func Must(err error) {

common/herrors/errors_test.go

@@ -14,6 +14,7 @@
 package herrors
 
 import (
+	"errors"
 	"fmt"
 	"testing"
 
@@ -34,3 +35,12 @@ func TestIsNotExist(t *testing.T) {
 	// os.IsNotExist returns false for wrapped errors.
 	c.Assert(IsNotExist(fmt.Errorf("foo: %w", afero.ErrFileNotFound)), qt.Equals, true)
 }
+
+func TestIsFeatureNotAvailableError(t *testing.T) {
+	c := qt.New(t)
+
+	c.Assert(IsFeatureNotAvailableError(ErrFeatureNotAvailable), qt.Equals, true)
+	c.Assert(IsFeatureNotAvailableError(&FeatureNotAvailableError{}), qt.Equals, true)
+	c.Assert(IsFeatureNotAvailableError(errors.New("asdf")), qt.Equals, false)
+
+}

config/commonConfig.go

@@ -148,7 +148,7 @@ func (b BuildConfig) UseResourceCache(err error) bool {
 	}
 
 	if b.UseResourceCacheWhen == "fallback" {
-		return err == herrors.ErrFeatureNotAvailable
+		return herrors.IsFeatureNotAvailableError(err)
 	}
 
 	return true

resources/resource_transformers/babel/babel.go

@@ -181,7 +181,7 @@ func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx
 	if err != nil {
 		if hexec.IsNotFound(err) {
 			// This may be on a CI server etc. Will fall back to pre-built assets.
-			return herrors.ErrFeatureNotAvailable
+			return &herrors.FeatureNotAvailableError{Cause: err}
 		}
 		return err
 	}
@@ -200,7 +200,7 @@ func (t *babelTransformation) Transform(ctx *resources.ResourceTransformationCtx
 	err = cmd.Run()
 	if err != nil {
 		if hexec.IsNotFound(err) {
-			return herrors.ErrFeatureNotAvailable
+			return &herrors.FeatureNotAvailableError{Cause: err}
 		}
 		return fmt.Errorf(errBuf.String()+": %w", err)
 	}

resources/resource_transformers/postcss/integration_test.go

@@ -168,6 +168,25 @@ func TestTransformPostCSSError(t *testing.T) {
 
 }
 
+func TestTransformPostCSSNotInstalledError(t *testing.T) {
+	if !htesting.IsCI() {
+		t.Skip("Skip long running test when running locally")
+	}
+
+	c := qt.New(t)
+
+	s, err := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           c,
+			NeedsOsFS:   true,
+			TxtarString: postCSSIntegrationTestFiles,
+		}).BuildE()
+
+	s.AssertIsFileError(err)
+	c.Assert(err.Error(), qt.Contains, `binary with name "npx" not found`)
+
+}
+
 // #9895
 func TestTransformPostCSSImportError(t *testing.T) {
 	if !htesting.IsCI() {

resources/resource_transformers/postcss/postcss.go

@@ -205,7 +205,7 @@ func (t *postcssTransformation) Transform(ctx *resources.ResourceTransformationC
 	if err != nil {
 		if hexec.IsNotFound(err) {
 			// This may be on a CI server etc. Will fall back to pre-built assets.
-			return herrors.ErrFeatureNotAvailable
+			return &herrors.FeatureNotAvailableError{Cause: err}
 		}
 		return err
 	}
@@ -240,7 +240,9 @@ func (t *postcssTransformation) Transform(ctx *resources.ResourceTransformationC
 	err = cmd.Run()
 	if err != nil {
 		if hexec.IsNotFound(err) {
-			return herrors.ErrFeatureNotAvailable
+			return &herrors.FeatureNotAvailableError{
+				Cause: err,
+			}
 		}
 		return imp.toFileError(errBuf.String())
 	}

resources/transform.go

@@ -449,7 +449,7 @@ func (r *resourceAdapter) transform(publish, setContent bool) error {
 		newErr := func(err error) error {
 			msg := fmt.Sprintf("%s: failed to transform %q (%s)", strings.ToUpper(tr.Key().Name), tctx.InPath, tctx.InMediaType.Type)
 
-			if err == herrors.ErrFeatureNotAvailable {
+			if herrors.IsFeatureNotAvailableError(err) {
 				var errMsg string
 				if tr.Key().Name == "postcss" {
 					// This transformation is not available in this