If a Transformer throws a TransformationFailedException, the details of this exception are lost.

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Form.php line 592:

        } catch (TransformationFailedException $e) {
            $this->synchronized = false;
        }

The exception code+message should probably be kept and displayed.

Instead, the error becomes "This value is not valid.". I've not been able to find out how, but this message occurs in:
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/Validator/Type/FormTypeValidatorExtension.php

Related:
#4860
#703