gaiacli: Improve the error message for invalid passphrases

[ValarDragon]

Ref #1497

• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG.md
• Updated Gaia/Examples
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)
• Added appropriate labels to PR (ex. wip, ready-for-review, docs)

[codecov]

Codecov Report

Merging #1498 into develop will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1498      +/-   ##
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+ Coverage     64.4%   64.48%   +0.07%     
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  Files          118      118              
  Lines         6496     6493       -3     
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+ Hits          4184     4187       +3     
+ Misses        2062     2050      -12     
- Partials       250      256       +6

[cwgoes]

Can we just change this error in the keybase itself? That code should be in the SDK now.

[ValarDragon]

This error is in the armor file, for the actual symmetric encryption/decryption so its still in tendermint. I'm not sure this is the error message we want to be using for tendermint usecases though. (tendermint/tendermint#1821)

I think the better solution though is to eventually have an error table which maps from error key to error message, which everyone uses. (Would also make multi-language support easier in the future)

[cwgoes]

I'm not a fan of checking and rewriting an error message like this; it will just introduce more work in the future when we change it upstream. Can we submit a PR to Tendermint to use the error message we want instead (or use an error table, as you mention)?