Present meaningful message to user when backend is unreachable

[grzanka]

I've tried to check what happens if frontend it working, but backend is dead and users tries to login via plgrid on yap-dev.
The message to the user is meaningless in that case. See movie below

Screencast.from.2024-10-17.15-42-12.webm

[grzanka]

Backend can be unreachable for couple of reasons: it can be dead or connection can be blocked by firewall.
We saw today second reason. User could reach https://yaptide.c3.plgrid.pl/ and use the UI, when logging via plgrid user got redirected to keycloak. Unfortunately later got a following error:

After some inspection we realized that user was using "Guest WiFi" network in IFJ PAN. In this network also an attempt to connect to https://yaptide.c3.plgrid.pl:8443/ (this endpoint returns hello world and doesn't require authentication) failed.

I suggest to add much clearer indication whether backend is reachable or not. If backend is not reachable it doesn't make sense to let the user through logging process.