Improve "keypair data is invalid" error message

When doing a keypair-add, I was hitting a permission denied issue with
ssh-keygen -l. This turned out to be a configuration issue, but the
error message on the client side was pretty unhelpful with debugging
the issue:

  BadRequest: Keypair data is invalid (HTTP 400)

Fix that so we get:

  BadRequest: Keypair data is invalid: failed to generate fingerprint (HTTP 400)

Also fix the other uses of InvalidKeypair so we include the general
"keypair data is invalid" prefix.

Change-Id: I209e76adaf7c18a9f2f2dbb6da62db6c1cd9461e
Merge "Improve "keypair data is invalid" error message"