include all errors on doc when validation fails.

No reason to only include one validation error, since we validate the
entire form we should show all the errors.  Then we can decide with to
send them all or not.  For now keeping as is - sending one message that
is all error messages concatenated.  Also, each invalid field will also
have a unique error code now, `invalid_<field-name>`.

TODO add a setting to determine how to handle validation/error
responses. Whether to send them all, concatenate or only send the first
one.