Add an interface for Transaction error code: [ECR-2006]

[dmitry-timofeev]

Overview

That's a quick prototype I decided to try when updating the documentation section on transaction exceptions: exonum/exonum-doc@9ce5998f553249897b0d7d3d8d5aae12bc20400e

Whilst it might appear as some extra work, it makes it easier to use an enum for the error codes instead of just a bunch of constants, which has some extra useful properties (like free #toString implementation).

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See: https://jira.bf.local/browse/ECR-XYZ

Definition of Done

• [ ] There are no TODOs left in the code
• [ ] Change is covered by automated tests
• [ ] The coding guidelines are followed
• [ ] Public API has Javadoc
• [ ] Method preconditions are checked and documented in the Javadoc of the method
• [ ] Changelog is updated if needed (in case of notable or breaking changes)
• [ ] The continuous integration build passes

[coveralls]

Coverage decreased (-0.05%) to 85.895% when pulling 5d7b8073807ed03991cc8559c69292c221024175 on dmitry-timofeev:add-interface-for-error-code into 4a8fabb8e22f7aa483d823447335657e4c172fd2 on exonum:master.