ensure EncodingPatternConverter#handlesThrowable is implemented

Guess the title is enough to understand what happens .
Long story short the handlesThrowable() logic to skip forced (by default) append of the exception does not work as soon as encoding is used which is almost always the case when outputing JSON/XML using PatternFormatter (since Json appender required 3rd party libs which are often undesired).
This PR implements this method by checking the nested formatters.