All exceptions extending from the RaidenError.
All exceptions extending from the RaidenError.

- Changed all exceptions to raise from a known base class, to easy a
try-catch-all for raiden exceptions.
- Added docstrings with a little explaination of the exception usage.

tomashaber/raiden,hackaugusto/raiden,tomashaber/raiden,tomashaber/raiden,hackaugusto/raiden,tomashaber/raiden,tomashaber/raiden