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
feat(API): Return error entities with unit 406s
fix(API): Serialize nil error properly
feat(API): Return error entities with unit 500s
test(API): Check ctype in assertErrorResponse
feat(API): Return error entities from all 404s
feat(API): Always send error entities from machines resource
Merge pull request #571 from bcwaldon/unit-errors

Moar error entities!
refactor(API): Use custom errorResponse struct

The googleapi pkg provides an Error entity, but it
unnecessaryily serializes a field we do not use.
Creating an equivalent struct that we maintain
works around the problem while retaining the ability
for the client bindings to use googleapi.Error.