handle file upload errors

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | -
| Tests pass?   | -
| Fixed tickets | #79
| License       | MIT
| Doc PR        | -

When uploading files through a form, there is no error handling. I thought we could use the upload file helper from the form type as well. This at least now gives a message that something went wrong and makes the upload file field marked as the field with error. however, it only gives the generic "invalid value" hint and does not show the ecxeption i throw. is there a way to better control that and give the exact error message like "file too big"?