Catch Docker API errors

Magnum container REST API hangs currently when there is an
error from Docker API ('ValueError: Circular reference detected').
So we should catch this APIError and extract the error string to
make sure the oslo messaging call returns properly (and hence the
magnum REST API call does not time out).

Change-Id: I055ffc2c733bb5193191a764169388369b5b1dcc
Merge "Catch Docker API errors"