[OpenDataServices/cove#708] Don't 500 error for fields not in the shcema
[OpenDataServices/cove#708] Don't 500 error for fields not in the shcema

(In sorting, append them at the end instead, this will then be picked up
as wrong in schema validaiton).

This is a copy of
https://github.com/OpenDataServices/cove/commit/57bbfc937f0bdcbaaf2047d84d9ffd8c8e27d806#diff-7698abd96129b324de44d434c901356e

OpenDataServices/flatten-tool