Currently most errors during creating posix.environ are ignored except an error of creating an empty dict.

The initial revision 85a5fbbdfea617f6cc8fae82c9e8c2b5c424436d contained the comment "XXX This part ignores errors". Later changes removed "XXX" from the comment and added explicit error clearing. Later the POSIX code was duplicated for Windows. It looks to me that that comment was not declared the intentional behavior, but just described existing code, and was left as a reminder for implementing error handling.

The proposed PR implements proper error handling in this code.