Revert with error message when release validation fails

[cgewecke]

13, #15.

• modifies the ReleaseValidator contract to return an integer error code if validation fails (current behavior is to return boolean)
• defines an integer code -> human readable message mapping in the validator contract.
• modifies PackageIndex to revert with error message when release validation fails (current behavior is to silently succeed without executing the release.)
• update tests to check for revert and appropriate error message.

This PR targets the allow-backfilling branch because they touch the same code / code has been removed in that PR.

NB: it might be cheaper to have the validator just return the strings? Not sure this implemented optimally at all. . .

[coveralls]

Pull Request Test Coverage Report for Build 105

• 9 of 9 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 97.175%

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Totals
Change from base Build 87:	0.4%
Covered Lines:	385
Relevant Lines:	391

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💛 - Coveralls