improve error handling when adding webauthn entries

improve error handling when adding webauthn entries

There are cases where we directly throw a string error
(particularly, when navigator.credentials.create() fails,
for example when trying to register the same WA device to
the same user twice), which would end up with the WA window
simply not closing before.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>