patch 8.2.3783: confusing error for using a variable as a function

Problem:    Confusing error for using a variable as a function.
Solution:   If a function is not found but there is a variable, give a more
            useful error. (issue #9310)

src/testdir/test_functions.vim

@@ -2234,6 +2234,12 @@ func Test_call()
   call call(test_null_partial(), [])
   call assert_fails('call test_null_function()()', 'E1192:')
   call assert_fails('call test_null_partial()()', 'E117:')
+
+  let lines =<< trim END
+      let Time = 'localtime'
+      call Time()
+  END
+  CheckScriptFailure(lines, 'E1085:')
 endfunc
 
 func Test_char2nr()