Exception handling

This MR introduces the handle_output argument in the slk_call function. This will give python wrappers the freedom to handle errorcodes themselves and give some more pythonic output. Examples are exists and iscached which should return True/False and only raise Exceptions if output.errcode > 1.

• slk_exists_bool will now either return False or the id if the file exists, exceptions are handled in slk_exists.
• slk_iscached_bool will return bool, exepctions are handled in slk_iscached.

closes #36 (closed)