Clarify CLA requirements and Gerrit error
Clarify CLA requirements and Gerrit error

Call out the error that Gerrit returns if there is no CLA on file.

PiperOrigin-RevId: 214964718
Change-Id: I3d92e3eb73f178e8c4c52b5defbe8d21db536215

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor