Slight refactoring of state check in pg_upgrade check_ function

I noticed that the pg_upgrade check_ functions were determining failures found
in a few different ways. Some keep a boolen flag variable, and some (like
check_for_incompatible_polymorphics) check the state of the script filehandle
which is guaranteed to be set (with the error message referring to the path of
said file). Others like check_loadable_libraries only check the flag variable
and fclose the handle assuming it was opened.

The attached diff changes the functions to do it consistently in one way, by
checking the state of the filehandle. Since we are referring to the file by
path in the printed error message it seemed the cleanest approach, and it saves
a few lines of code without IMO reducing readability.

There is no change in functionality, just code consistency.

--
Daniel Gustafsson https://vmware.com/