server response to 500 error is unhelpful

Currently, a 500 error on the server results in the server returning a message body that includes str(e), where e is the exception object. This can lead to leaking internal server details (bad), and very confusing messages. For example, when the server had an exception that was a "KeyError: 'id'", the server returned;

EBUG: quantumclient.client RESP BODY:{"QuantumError": "'id'"}

This results in very confusing output, for example, an attempt to create a floating IP which failed with the above message body appears to the user as:

nicira@com-dev:~/devstack$ quantum floatingip-create e7ad2c17-1cca-41fb-bb03-2e0b2b83703c --port_id e2b3a272-2bb7-486b-872b-38d11646d19b
'id'

That is, it just prints "id" and that's it.