Include exception message in UnexpectedError error frame

[prashantv]

Right now, the error frame description does not include any information about what failed:
https://github.com/uber/tchannel-python/blob/master/tchannel/tornado/dispatch.py#L231

We should consider adding e.message to the error frame for more information

cc @jcorbin

[jc-fireball]

Hard to tell whether want to pass the stack trace into the error message and send back to client. On one hand the exception message may disclosure some internal information from server which the server owner may not want to share it with all clients (like http internal server error). On the other hand by giving more error information, client can know more about what's happening.