web admin: better handling of invalid Fingerbank key

[nqb]

An invalid Fingerbank key is not correctly handle when saved.

To Reproduce
Steps to reproduce the behavior:

1. Go to Fingerbank -> General settings
2. Enter your API key: Zayme
3. Save
   => Next four API calls reply with 200 (upstream/collector/query/proxy)
4. Leave this menu and come back
   => your API key has disappeared, API call to account_info return:

{"errors":[],"message":"Invalid key","status":401}

but /usr/local/fingerbank/conf/fingerbank.conf contains your API key.

Expected behavior
Have an error message when your key is not valid.

[nqb]

Certainly related to #4805

[julsemaan]

#4805 currently prevents me from working on this issue since the settings section is broken in the admin