describe how to access form errors
feature #4168 [Components][Form] describe how to access form errors (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[Components][Form] describe how to access form errors

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

The ``getErrors()`` and ``getErrorsAsString()`` methods haven't been documented before. This pull request adds a short description for them. This forms as basis for the solution of #3660.

Commits
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5deccd2 describe how to access form errors