Fix eth_getTransactionReceipt throwing error

[amaury1093]

On parity-ethereum, api.pollMethod('eth_getTransactionReceipt') didn't throw if there was no receipt yet. Now it throws.

So we manually poll/catch for 20s.

[coveralls]

Pull Request Test Coverage Report for Build 172

• 5 of 8 (62.5%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 46.515%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/light.js/src/rpc/other/post.ts	5	8	62.5%

Files with Coverage Reduction	New Missed Lines	%
packages/light.js/src/rpc/other/post.ts	2	55.0%

Totals
Change from base Build 169:	-0.07%
Covered Lines:	840
Relevant Lines:	1805

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💛 - Coveralls