Prevent abort() call while parsing an input strings with too deep nestings

[smallSwed]

When JSON_USE_EXCEPTION set 0 jsoncpp should not abort when the input string contains object/array which is exceeds the stackLimit.
The caller unable to validate that the input string exceeds the nesting stackLimit without parsing the json, so abort is a bad response for this case.
When the JSON_USE_EXCEPTION is 1 then the caller can catch the thrown exception and handle the issue how they want it.

With this change I intend to make the non-exception variant case fail gracefully too and handle-able.

[smallSwed]

I wasn't sure to how to rework the error recovery, but maybe in this case the parser may better to skip the whole recovering all together.
I think that could be achieved by the code chage when the stackLimit exceeded:

    ...
    current_ = end_; // short circuit the parsing to prevent error recovery
    return addError(
    ...