Use a more proper error kind for quota conflicts, supporting the error type
*servicestate.QuotaChangeConflictError inside errToResponse and indicating what
sort of change the conflict is being used for.
Also update the unit tests for the daemon error response to check for this.
Signed-off-by: Ian Johnson <ian.johnson@canonical.com>
apiError is a StructuredResponse (and also an error), the plan is to
use it everywhere instead of building the complicated respJSON for
errors directly
it can also be used as a return type for functions that are meant
exactly to build errors
