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/main/views/webauthn_credentials.py

@@ -3,7 +3,7 @@ from flask import current_app, request, session
 from flask_login import current_user
 
 from app.main import main
-from app.models.webauthn_credential import WebAuthnCredential
+from app.models.webauthn_credential import RegistrationError, WebAuthnCredential
 from app.notify_client.user_api_client import user_api_client
 from app.utils import user_is_platform_admin
 
@@ -34,13 +34,19 @@ def webauthn_begin_register():
 @main.route('/webauthn/register', methods=['POST'])
 @user_is_platform_admin
 def webauthn_complete_register():
-    credential = WebAuthnCredential.from_registration(
-        session.pop("webauthn_registration_state"),
-        cbor.decode(request.get_data()),
-    )
+    if 'webauthn_registration_state' not in session:
+        return cbor.encode("No registration in progress"), 400
+
+    try:
+        credential = WebAuthnCredential.from_registration(
+            session.pop("webauthn_registration_state"),
+            cbor.decode(request.get_data()),
+        )
+    except RegistrationError as e:
+        return cbor.encode(str(e)), 400
 
     user_api_client.create_webauthn_credential_for_user(
         current_user.id, credential
     )
 
-    return ''
+    return cbor.encode('')