fix(API): Send correct Content-Type with error entities
Merge pull request #573 from bcwaldon/fix-error-ctype

fix(API): Send correct Content-Type with error entities
refactor(API): Ensure all unit fields exposed
Merge pull request #574 from bcwaldon/unit-fields

refactor(API): Ensure all unit fields exposed
feat(API): Send errors with PUT /units/<name> 400s
feat(API): Send errors with DELETE /units/<name> 400s
feat(API): Send error with GET /units/<name> 404