Tidy style & docs in catchError

[tanem]

Keeping things consistent with the other files I've been looking at 💄

[coveralls]

Pull Request Test Coverage Report for Build 8082

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.2%) to 96.962%

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Files with Coverage Reduction	New Missed Lines	%
src/internal/observable/fromPromise.ts	1	87.5%
src/internal/observable/fromIterable.ts	2	95.24%
src/internal/observable/fromObservable.ts	2	83.33%

Totals
Change from base Build 8066:	0.2%
Covered Lines:	5777
Relevant Lines:	5958

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💛 - Coveralls

[benlesh]

Thank you, @tanem