[BUG] Error handling when application is crashing on root + general improvement of error handling

[danieldanielecki]

Describe the bug
When application is crashing on the root level, a second screenshot from #15 might be displayed.

To Reproduce
Steps to reproduce the behavior:

1. Simulate crashing of entire application.

Expected behavior
Error handling is done nicely for exam & practice tests, that's good, with nice UI. Also for search queries, in page.tsx, but something on the root level could've been added as well. The app can display the same error page like for exam & practice tests.

Screenshots

Desktop (please complete the following information):

• OS: any
• Browser: any
• Version: any (discovered in 1.2.0)

Smartphone (please complete the following information):

• Device: any
• OS: any
• Browser: any
• Version: any (discovered in 1.2.0)

Additional context
Small prio, but good first issue.

[danieldanielecki]

After testing: in general, error handling doesn't work perfectly for practice & exam pages. questionSet object needs to be debugged if we can rely on this in displaying error (if (!questionSet) { check)`.

Additionally, for practice test there must be also error handling on each individual question due to having an unique URL.