Make ExternalName.SetIdentifierArgumentFn return error

[muvaf]

What problem are you facing?

SetIdentifierArgumentFn does not return an error since it's a simple assignment because in most of the cases we need to assign to a top level. However, more advanced functions setting value on a field path have the possibility of failure, which needs to be returned as error rather than panic since that function is called only in runtime - not generation time.

How could Terrajet help solve your problem?

Change the signature to return error.