Fix function call type error message

src/interpret.rs

@@ -260,7 +260,11 @@ impl ExecEnv {
                     self.fn_call(func, &args, &stmt.span)?;
                 } else {
                     return Err(Error {
-                        kind: ErrorKind::TypeError(iden.iden.to_owned()),
+                        kind: ErrorKind::TypeError(format!(
+                            "attempt to call non-function `{}` (= {})",
+                            iden.iden.to_owned(),
+                            func
+                        )),
                         span: stmt.span.clone(),
                     });
                 }