Allow to retrieve bindHook errors

The original error returned by bindHook is currently lost.
I have use cases where I need to access this error details in order to
return specific error messages to end users.

This commit adds the ability to retrieve the bindHook error from the
BindError struct.

Author: mcorbin

tonic/handler.go

@@ -73,7 +73,7 @@ func Handler(h interface{}, status int, options ...func(*Route)) gin.HandlerFunc
 			input := reflect.New(in)
 			// Bind the body with the hook.
 			if err := bindHook(c, input.Interface()); err != nil {
-				handleError(c, BindError{message: err.Error(), typ: in})
+				handleError(c, BindError{message: err.Error(), typ: in, bindHookErr: err})
 				return
 			}
 			// Bind query-parameters.

tonic/tonic.go

@@ -230,6 +230,7 @@ func Tags(tags []string) func(*Route) {
 // to bind parameters, to differentiate from errors returned
 // by the handlers.
 type BindError struct {
+	bindHookErr   error
 	validationErr error
 	message       string
 	typ           reflect.Type
@@ -258,6 +259,11 @@ func (be BindError) ValidationErrors() validator.ValidationErrors {
 	return nil
 }
 
+// BindHookError returns the error from the bind process.
+func (be BindError) BindHookError() error {
+	return be.bindHookErr
+}
+
 // An extractorFunc extracts data from a gin context according to
 // parameters specified in a field tag.
 type extractor func(*gin.Context, string) (string, []string, error)