Detect when the explorer is down

[Tommytrg]

Detect when the explorer is down and show a modal message in the app

[aesedepece]

In order not to make things worse regarding #184, I'd suggest simply showing an alert in any form always that a query to the explorer backend fails or times out.

UI wise, a modal should just work, although my preferred way would be to have a sort of sticky red toast come up from the bottom edge of the screen and stay there with a message saying "You are having connection issues" and no way to close dismiss it. Once a subsequent query succeeds, this red toast would be replaced with another one in green that reads "Connection reestablished!" and allows dismissal.

It can probably made prettier, but this is roughly the idea:

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The dismiss button could also be replaced with the typical [X] thingy.