This CL updates the implementation of PostReview to replace
all instances of raising UnprocessableEntityException
(HTTP 422) with failed Additions instead. This matches the
docs for Set Reviewer, which no longer mention 422 being a
valid response code. This also falls in line with
RFC 7231, which promotes HTTP 400s with detailed bodies
over 422s.
It also updates the documentation to be clearer about
how these API functions work.
Bug: issue 6016
Change-Id: I1adad98107ba021132293ec47c6a6da01787556e
8c650215eb