Improve error messages for PostCSS etc.

Fixes #9730

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())
 	}