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) {