test_support._is_gui_available is now defined the same way on every
platform, and now includes the Windows-specific check that had been in the
Windows version of _is_gui_available and the OSX-specific check that was
in runtktests.check_tk_availability. Also, every platform checks whether
Tk can be instantiated (if the platform-specific checks passed).
return the error message produced by importlib, so that if an import
in the package whose import is being wrapped is what failed the skip
message will contain the name of that module instead of the name of the
wrapped module. Also fixed formatting of some previous comments.
