Subquery in 'expression' JQL function isn't necessarily valid when expression script is being validated

To reproduce:

• Go to the issue navigator in Advanced mode and enter a JQL query using the expression function with a syntactically incorrect subquery, e.g.:

  issueFunction in expression("project = A B", "reporter == assignee")
  

  (The error in the above is that project names with whitespace need to be enclosed in quotes.)

• Submit the query.

Expected: A syntax error message appears for the subquery.

Actual: A 'compilation failure' error message appears, quoting the syntax error.