Staking dapp: unhealthy state message

[vbaranov]

Motivation

Display error if staking dapp is unhealthy (including a partial case of archive node is unhealthy).

Changelog

Display error message if epoch_end_block is equal to 0.

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 97a1559cb415e07c2bfa9428dabd131e48f85a4b-PR-3565

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 159 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.1%) to 66.613%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/channels/stakes_channel.ex	159	0%

Totals
Change from base Build d4d1a591992b0fc100d7ec3b82674da1079a8727:	-1.1%
Covered Lines:	2073
Relevant Lines:	3112

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💛 - Coveralls