Specification: error modes on unsupported attributes

[andre-merzky]

Describe the error, inconsistency, or unclear wording here
The spec allows to set custom attributes, but is silent on what error conditions can occur, e.g., if an attribute is not supported by the implementation. Note that the validity of the attributes may only become clear once a job is being submitted to an JobExecutor instance.

Suggested Clarification

Clarify behavior on unsupported / invalid attributes.