Handle bad gateway at pending transactions fetcher

[vbaranov]

Motivation

2021-03-21T20:59:55.941 fetcher=pending_transaction [error] Task #PID<0.13429.13> started from Indexer.Fetcher.PendingTransaction terminating
** (CaseClauseError) no case clause matching: {:error, {:bad_gateway, "https://xdai-archive-df.xdaichain.com"}}
(indexer 0.1.0) lib/indexer/fetcher/pending_transaction.ex:120: Indexer.Fetcher.PendingTransaction.task/1
(elixir 1.11.3) lib/task/supervised.ex:90: Task.Supervised.invoke_mfa/2
(elixir 1.11.3) lib/task/supervised.ex:35: Task.Supervised.reply/5
(stdlib 3.14) proc_lib.erl:226: :proc_lib.init_p_do_apply/3
Function: #Function<2.24088951/0 in Indexer.Fetcher.PendingTransaction.handle_info/2>
Args: []

Changelog

Handle {:error, {:bad_gateway, _}} response in pending transactions fetcher

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 444f2c7451962f0da5989398fceb7bc1b2585f1d-PR-3731

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 64.641%

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Totals
Change from base Build b3dd8a8543b2120f9df1e47a6a025afac8ae5883:	0.4%
Covered Lines:	2117
Relevant Lines:	3275

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💛 - Coveralls