chore: Replace fmt.Errorf to errors.Errorf

[developerfred]

close #236

ref
https://blog.golang.org/go1.13-errors
https://golang.org/pkg/errors/

[coveralls]

Pull Request Test Coverage Report for Build 370818079

• 3 of 107 (2.8%) changed or added relevant lines in 21 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 36.643%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/ops/miningManager.go	0	1	0.0%
pkg/pow/miningSetup.go	0	1	0.0%
pkg/rest/server.go	0	1	0.0%
pkg/rpc/contractRPC.go	0	1	0.0%
pkg/tracker/retryFetcher.go	1	2	50.0%
pkg/tracker/valueGenerators.go	0	1	0.0%
pkg/util/logger.go	0	1	0.0%
pkg/db/localDataProxy.go	0	2	0.0%
pkg/pow/pool.go	0	2	0.0%
pkg/tracker/runner.go	0	2	0.0%

Totals
Change from base Build 370303183:	0.05%
Covered Lines:	1598
Relevant Lines:	4361

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💛 - Coveralls