Steps to reproduce
In the FEEL Scala Engine, use a single-quoted string literal in a unary test (which is not allowed by the specification), e. g. 'asdfasdf'.

Observed behavior
The following error message is shown:

failed to evaluate expression 'asdfasdf': no variable found for name 'asdfasdf'

Expected behavior
A more expressive error message is shown that the user can easier understand that a single-quoted string literal is not allowed and must be replaced by a double-quoted string literal.