UNOMI-846: improve log for event validation

[jsinovassin]

https://issues.apache.org/jira/browse/UNOMI-846

[jkevan]

Despite the fact that I think changing this log levels directy in the code is a bad idea, mostly because of:

• security
• amount of log printed by default
• the fact that normally log level is configurable (should be configurable) outside the code.

There is inconsistent logs, I wonder why sometimes for a ValidationException we now sometimes log it as:

 LOGGER.warn("An event was not valid - switch to DEBUG log level for more information.");
            LOGGER.debug("{}", e.getMessage(), e);

and sometimes like this:

 LOGGER.warn("{}", e.getMessage(), e);

Also I am not really sure about the wording in the message: "An event was not valid" in case of ValidationException, to me it's more a crash/error before/during the validation. Which is different than schema validation results.
I would rename the log to: "An error occurred during event validation", to make that difference, if the goal of thoses logs are to be clear for developers to have useful/understandable informations, they should be clear and consistent.

[jkevan]

LOGGER.isDebugEnabled() should be removed