Externalize error messages in the v2 API

This patch does more internationalization for the REST API error
messages that don't currently have it to take advantage of the new
support added by bp user-locale-api to show error messages in the locale
requested by the user through the Accept-Language HTTP header.

We only do v2 because consumers have used the response error message in
the past for error checks, so changing it in v1 too would break them.

Partially implements bp user-locale-api

Change-Id: I92780b42c125a91ab4916b7a31e4b71d306a89a1
Merge "Externalize error messages in the v2 API"