make source accessible (#792)

Author: scottlepp

experimental/errorsource/error_source.go

@@ -28,6 +28,11 @@ func (r Error) Unwrap() error {
 	return r.err
 }
 
+// Source provides the error source
+func (r Error) Source() backend.ErrorSource {
+	return r.source
+}
+
 // PluginError will apply the source as plugin
 func PluginError(err error, override bool) error {
 	return SourceError(backend.ErrorSourcePlugin, err, override)