Use different fallback for old Vims

Reasoning: currently used fallback works well only if relatively recent patches 
are there: specifically the one that transforms Vim errors to Python exceptions.

This variant should work in any case, but it has a downside: it does not test 
whether function exists, it tests whether argument given to vim_func_exists 
denote some callable object (which may as well be global variable with the same 
name). When it comes to CapsLockStatusline I do not care much as I am using 
`vim.eval` to call it and not saving reference to this function somewhere.

Fixes #1146