Descriptive message for KeyError, still 500

[hkethi002]

Fixes #918 Example

{
  "status_code": 500,
  "message": "Key 'stordage' was not found",
  "request_id": "e6f7b979-1511802434"
}

Review Checklist

• Tests were added to cover all code changes
• Documentation was added / updated
• Code and tests follow standards in CONTRIBUTING.md

[codecov-io]

Codecov Report

Merging #1011 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1011      +/-   ##
==========================================
+ Coverage   90.43%   90.43%   +<.01%     
==========================================
  Files          50       50              
  Lines        6753     6756       +3     
==========================================
+ Hits         6107     6110       +3     
  Misses        646      646

[nagem]

LGTM