Horizon throws a generic error when failing to import a public key

When a user attempts to import a keypair into Horizon, and the import fails, they are faced with a generic error message: "unable to import keypair".

This does not help the user in identifying what they may have done wrong (invalid name, invalid, maybe they even mistakenly pasted a private key in there).

Additionally, this doesn't help the user identify whether they made the mistake (this would be a Bad Request), or whether there's an issue with nova that their administrator should be looking at.

Since the import is done through novaclient, we can get use the error message from Nova when faced with a Bad Request, and fall back to the generic error message in case we received another error.

Here is a proposed fix: https://review.openstack.org/#/c/67334/