Describe the bug
During the last few days, QLever backend was very regularly on/off (no idea why).
This leads to the queries failing but without any additional information so people simply get the feeling things don't work period.

To Reproduce
Steps to reproduce the behavior:
Hard to do, the backend unavailability has been quite idiocratic

Expected behavior
A clear message saying the reason the query failed is because of backend accessibility and this is a temporary issue rather than a persistent one

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.