Logs only mode

[eterey]

Provide a possibility to turn on the "logs only" mode, which will work in the following way:

1. Validation exceptions won't be raised anymore;
2. The pyvalid will keep working as usual and will keep validating input/output values;
3. In case of validation error, the warning (type of message TBD) message will be written.

It may be helpful for production mode, when any runtime errors are unwelcome.

[Aparna-BS]

I can work on this enhancement.

[eterey]

It would be great, thanks @Aparna-BS !