Improve STC error message for non-functional interface

When the STC decides that a method reference is being provided for a type that is not a functional interface, it prints the error

The argument is a method reference, but the parameter type is not a functional interface

without specifying what it believes the parameter type is. It would be helpful for troubleshooting to identify the parameter type.

https://github.com/groovy/groovy-eclipse/blob/c2999392e161037435c9479bdb2f71f70a3c0f8b/base/org.codehaus.groovy40/src/org/codehaus/groovy/transform/stc/StaticTypeCheckingVisitor.java#L3665