Handle errors when registration fails

Previously we would raise a 500 error in a variety of cases:

- If a second key was being registered simultaneously (e.g. in a
separate tab), which means the registration state could be missing
after the first registration completes. That smells like an attack.

- If the server-side verification failed e.g. origin verification,
challenge verification, etc. The library seems to use 'ValueError'
for all such errors [1] (after auditing its 'raise' statements, and
excluding AttestationError [2], since we're not doing that).

- If a key is used that attempts to sign with an unsupported
algorithm. This would normally raise a NotImplemented error as part
of verifying attestation [3], but we don't do that, so we need to
verify the algorithm is supported by the library manually.

This adds error handling to return a 400 response and error message
in these cases, since the error is not unexpected (i.e. not a 500).
A 400 seems more appropriate than a 403, since in many cases it's
not clear if the request data is valid.

I've used CBOR for the transport encoding, to match the successful
request / response encoding. Note that the ordering of then/catch
matters in JS - we don't want to catch our own throws!

[1]: 142587b3e6/fido2/server.py (L255)
[2]: c42d9628a4/fido2/attestation/base.py (L39)
[3]: c42d9628a4/fido2/cose.py (L92)

app/assets/javascripts/registerSecurityKey.js

@@ -27,7 +27,16 @@
             })
             .then((response) => {
               if (!response.ok) {
-                throw Error(response.statusText);
+                return response.arrayBuffer()
+                  .then((cbor) => {
+                    return Promise.resolve(window.CBOR.decode(cbor));
+                  })
+                  .catch(() => {
+                    throw Error(response.statusText);
+                  })
+                  .then((text) => {
+                    throw Error(text);
+                  });
               }
 
               window.location.reload();