Replace checks with modifiers

[fvictorio]

Part of the audit findings See BlockchainLabsNZ/poa-popa#1

Replace checks with modifiers.

[coveralls]

Pull Request Test Coverage Report for Build 394

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.4%) to 79.042%

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Totals
Change from base Build 392:	-0.4%
Covered Lines:	941
Relevant Lines:	1166

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💛 - Coveralls