Reset modError after process()

### What does it do ?

Resets `modError` after calling `modError::process()`

### Why is it needed ?

First, some background (as i understand things to work)

* processors make extensive usage of modError to indicate if any error occurred
* `modProcessor` derivative classes stop execution if `modError` has some error OR if `modError->message` is not empty (which occurs if you pass any message, even a successful one)

This prevents multiple processors to run one after the other if either a message is sent to `modError` or if any error occurs.
Sample case : multiple TVs making use of processors. If a processor fails, all remaining TVs processors will never be executed since `modProcessor::initialize` check for any "error".

This PR aims to "reset" `modError` after every `modError::process()` call.

I'm not sure that's the expected behavior about `modError` (ie. should it keep all errors during modX instance ? If so, `modProcessor::initialize()` logic should be modified).

Hopefully this makes sense (if not, i'll gladly try to reformulate)

### Related issue(s)/PR(s)

* #11951
* #11498 (PR which introduced the current behavior)