R4R: Introduce RESTful error responses

[alessio]

Make error responses turn into JSON responses.

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #3485 into develop will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           develop   #3485      +/-   ##
==========================================
- Coverage    59.42%   59.4%   -0.03%     
==========================================
  Files          130     130              
  Lines         9804    9804              
==========================================
- Hits          5826    5824       -2     
- Misses        3608    3610       +2     
  Partials       370     370

[alexanderbez]

LGTM -- left a minor comment that should be addressed before approval.

[cwgoes]

ACK