Validators correctly raise exceptions

[azmeuk]

This PR should fix #445
Because ValidationError inherits from ValueError, A ValueError raised inside a validator is just considered as the validator refusing the data. This behavior felt unexpected to me, and is fixed in this PR.

Here is the piece of code from #445 illustrating the bug:

>>> from wtforms import StringField, Form
>>> from werkzeug.datastructures import MultiDict
...
>>> def division_error_validator(form, field): raise ZeroDivisionError
>>> class DivisionErrorForm(Form):
...     foo = StringField(validators=[division_error_validator])
...
>>> def value_error_validator(form, field): raise ValueError
>>> class ValueErrorForm(Form):
...    foo = StringField(validators=[value_error_validator])
...
>>> DivisionErrorForm(MultiDict({"bar": "baz"})).validate()
Traceback (most recent call last):
...
ZeroDivisionError

>>> ValueErrorForm(MultiDict({"bar": "baz"})).validate()
Traceback (most recent call last):
  File "/home/azmeuk/dev/wtforms/src/wtforms/fields/core.py", line 275, in _run_validation_chain
    validator(form, self)
  File "<stdin>", line 1, in value_error_validator
ValueError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/azmeuk/dev/wtforms/src/wtforms/form.py", line 300, in validate
    return super(Form, self).validate(extra)
  File "/home/azmeuk/dev/wtforms/src/wtforms/form.py", line 136, in validate
    if not field.validate(self, extra):
  File "/home/azmeuk/dev/wtforms/src/wtforms/fields/core.py", line 255, in validate
    stop_validation = self._run_validation_chain(form, chain)
  File "/home/azmeuk/dev/wtforms/src/wtforms/fields/core.py", line 281, in _run_validation_chain
    self.errors.append(e.args[0])
IndexError: tuple index out of range

[davidism]

#61 is related

[davidism]

I'm not 100% sure on this one, I feel like there was a reason ValueError was accepted in certain places. Are there previous discussions about this?

[azmeuk]

A comment on the thread you mentioned, says that during the processing phase, ValidationError are unrelevant, so only ValueError are handled.

But I am not sure this is directly related to this patch, because this line it changes to except ValidationError, is in _run_validation_chain, so basically in the validation phase.
https://github.com/wtforms/wtforms/blob/129040ab4d5af83a05ddc7b63f854d48be3d55aa/src/wtforms/fields/core.py#L280

My feeling is that we should trigger special wtforms actions (such as making the validation fail (i.e. return False)) on wtforms exceptions, and let the other exception types reach the user so they can be debugged.
Currently, there is no way to differentiate a ValueError that is raised to make the validation fail and a classical ValueError.

edit: I edited the changelog, and made the behavior consistent with pre_validate and post_validate.

[azmeuk]

Thank you. I think I won't backport this one.