Catch `ServerDownOrUnreachableError` in redfish_available function (#… #62

Job	Run time
check / lint-unit (3.8) / Lint checkers and Unit tests	1m 17s
check / lint-unit (3.10) / Lint checkers and Unit tests	1m 11s
check / func (2.9/stable, FUNC_ARGS='--series focal' make functional) / Functional tests	13m 32s
check / func (3.1/stable, FUNC_ARGS='--series jammy' make functional31) / Functional tests	12m 38s
release / Release to CharmHub	2m 23s
	31m 1s

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