[WIP][Form] Improve transformation failure mesasge by providing property path

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   |
| Fixed tickets | #11287
| License       | MIT
| Doc PR        |
Interesting - it seems that this test class should have *always* thrown this (i.e. it was a minor bug in the test I think). Unless I'm missing something, nice work!