(new): rich message support

(fix): runtime reliability
(tests): regression coverage
(doc): v1.1 release notes

error_model.go

@@ -8,6 +8,7 @@ type classifiedError struct {
 	internalOnly bool
 }
 
+// Error returns the underlying error message.
 func (e *classifiedError) Error() string {
 	if e == nil || e.err == nil {
 		return ""
@@ -15,6 +16,7 @@ func (e *classifiedError) Error() string {
 	return e.err.Error()
 }
 
+// Unwrap returns the underlying error.
 func (e *classifiedError) Unwrap() error {
 	if e == nil {
 		return nil