Give warning when backend is not active

[merav-aharoni]

Summary

Give warning when the backend is not active in QiskitRuntimeService.run.

Details and comments

Fixes #884.

[coveralls]

Pull Request Test Coverage Report for Build 5215323344

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 72.612%

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Totals
Change from base Build 5213006080:	0.03%
Covered Lines:	2577
Relevant Lines:	3549

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💛 - Coveralls

[merav-aharoni]

@kt474 - do you think a test is necessary? I checked it locally on the real devices. But creating a test turned out to be complicated, because it involves calling run on a backend whose status needs to be manipulated to be non-active.