Small UnifiedAuthenticator refactor and other small changes

[netsettler]

This consolidates a bunch of smallish fixes I'd been accumulating on various branches for a while.

• Rewrite unified_authentication as a class UnifiedAuthenticator with class methods, and then set unified_authentication to one of those methods for compatible calling. This breaks up the reasons for the cascaded operations computing the result into named calls that have some meaning.

• Add function test_utc_now_str that was defined in another repo but belongs in misc_utils.

• New functionality for command_utils.script_catch_errors to manage script failures without suggestion of reporting problem.

[coveralls]

Pull Request Test Coverage Report for Build 4409884028

• 112 of 114 (98.25%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 74.147%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dcicutils/s3_utils.py	31	33	93.94%
<!--	Total:	112	114	98.25%	-->

Totals
Change from base Build 4409843276:	0.3%
Covered Lines:	6023
Relevant Lines:	8123

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💛 - Coveralls