btcjson: add new JSON-RPC errors and document them

[onyb]

Just some basic housekeeping - add new JSON-RPC errors and document all the exported error codes. See source.

[coveralls]

Pull Request Test Coverage Report for Build 297532456

Warning: This coverage report may be inaccurate.

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A quick fix for this PR: rebase it; your next report should be accurate.

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.002%) to 53.504%

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Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	2	76.18%

Totals
Change from base Build 289529546:	0.002%
Covered Lines:	20780
Relevant Lines:	38838

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💛 - Coveralls

[jcvernaleo]

OK