Better feedback on CMIS RecoverableClientException

A CMIS RecoverableClientException error should include a meaningful message in the response. Currently the Browser Binding response for a RecoverableClientException is:

"exception": "runtime"
"message": "error"

This is especially limiting now that the default Nuxeo CMIS behavior is to exclude the stacktrace property from the response. This change is due to the upstream fix for Apache Chemistry OpenCMIS issue CMIS-950.