backend sanitization methods should return "reason"s along with the error code.

[v0lkan]

along with the error code, also return some explanatory message

instead of this for example

err = validation.ValidateSpiffeIdPattern(spiffeIdPattern)
if err != nil {
responseBody := net.MarshalBody(reqres.PolicyCreateResponse{
Err: data.ErrBadInput,
}, w)
net.Respond(http.StatusBadRequest, responseBody, w)
return err
}

do this

err = validation.ValidateSpiffeIdPattern(spiffeIdPattern)
if err != nil {
responseBody := net.MarshalBody(reqres.PolicyCreateResponse{
Err: data.ErrBadInput,
Reason: "Invalid spiffe id pattern. Matcher should be a regex that can match a spiffe id"
}, w)
net.Respond(http.StatusBadRequest, responseBody, w)
return err
}

for all validations.