Account Types; Elements; validation issues fixed

The validation of Account Type Elements within the business layer has been tidied up and issues fixed.

Certain error messages had wording the wrong way round, and one piece of validation may have caused a runtime error under certain circumstances, e.g., if an invalid ElementPk was specified.

NOTE: This runtime error would never occur when using finPOWER Connect, only when configuring an Account Type via the business layer.