Fix various staticcheck warnings

[gfr10598]

This addresses all warnings from staticcheck ./... All code changes were done manually. After all code changes, go.mod, go.sum updated with

go mod tidy

There was an unchecked error at one point, so I added checking, but there is now an uncovered conditional line. 8-(

Tested only through travis. We should also deploy to staging, and ensure that it works there before releasing to prod. This will be somewhat problematic, since we are still running 0.6.0 in prod.

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This change is 

[SaiedKazemi]

Can you also describe how you tested these changes?

[coveralls]

Pull Request Test Coverage Report for Build 117

• 24 of 27 (88.89%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.4%) to 97.27%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/json.go	8	11	72.73%
<!--	Total:	24	27	88.89%	-->

Files with Coverage Reduction	New Missed Lines	%
parser/json.go	1	96.33%
<!--	Total:	1		-->

Totals
Change from base Build 111:	-0.4%
Covered Lines:	1069
Relevant Lines:	1099

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💛 - Coveralls