Handle substitution errors with no parameters to ValidationError

[jctanner]

% substituation requires all keys to be matched during substitution, so if a ValidationError happens to include a %(foo)s style variable not met by the parameters, it will throw a KeyError. In Field.run_validators there is also an accumulation of errors that are wrapped by a final ValidationError which can then throw TypeError if any of the sub-errors contain replaceable substrings.

This is more complicated than it first seemed. #9295 has a dateformat string throwing a ValueError, which won't ever work with foo % bar so this patch needs more testing and edge case handling.

This patch implements a new function to handle the string substitution and related errors in a unified way that returns the original string if something fails.

should fix #9295

[AlanCoding]

Catching these errors will unblock us from being able to upgrade. In the cases where the format errors, it is almost always the case that params is empty dict, so raising the error with the detail matches the expectation.

[tomchristie]

I'm not yet convinced by this... I'd suggest we instead revert #8863 as a first pass.
(Preventing breaking changes > introducing new functionality.)

The equivalent case in Django uses if error.params before using formatting, a revised attempt at #8863 which does the same ensuring that there's no chance of a regression when params are not used would be a good idea.

[auvipy]

@tomchristie #9326

[jctanner]

This is obsolete now that #9326 is merged, which I fully support. If someone re-attempts the feature in the future, please feel free to copy/paste the unit tests I made in this PR.