[Form] Documented the setInvalidMessage() method for data transformers

[javiereguiluz]

This fixes #11559.

[wouterj]

TransformationFailedException should be used when transformation failed. Data mappers shouldn't transform data, DataTransformers should. DataMappers should only map (the transformed) data to the subject (entity, DTO, array, etc).

So 👍 for documenting this feature, but please add it to the DataTransformer article to not advocate wrong usage

[javiereguiluz]

Wouter, thanks for the review ... I've reverted the changes to explain this in the data transformers article.

[wouterj]

Looks good. I'll merge later today, I want to do a minor reword while merging :)

[xabbuh]

@javiereguiluz Could you reword the commit message to talk about data transformers instead of data mappers? Might be less confusing for some future reader of the git history. :)

[xabbuh]

Ah, maybe that's not necessary. I edited the PR title and during merge we will squash anyway (which should take the PR title as the commit message if I am not mistaken)

[wouterj]

Thank you @javiereguiluz! I've added a bit more info on transformer errors in e33b5b2 and also used a slightly different reason to use exceptions: to show specific errors and include user information, instead of showing a static error.