Automatically show authentication UI if signing fails

[daguej]

It would be nice if tergent could trigger biometric authentication if signing fails, then retry sigining.

I've already done alias ssh='termux-fingerprint 1>/dev/null; ssh', but:

• You have to reauthenticate every time, regardless of the validity timeout
• ssh invoked from outside the shell won't authenticate

Should be as simple as extending the bridge to support api_method Fingerprint and call it if signing fails.