Add optional resource check and failback

[umputun]

the goal is to eliminate redirects to a node without the actual resource. the current version expects all nodes to have all the resources, and a not-found redirect is supposed to be handled by the node itself.

this PR adds an optional HEAD request prior to the final redirect and failed request will change the redirect to failback url + requested resource

[coveralls]

Pull Request Test Coverage Report for Build 1647223807

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.7%) to 86.667%

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Totals
Change from base Build 1647216784:	2.7%
Covered Lines:	221
Relevant Lines:	255

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💛 - Coveralls