pg_upgrade: fix CopyFile() on Windows to fail on file existence

Also fix getErrorText() to return the right error string on failure.
This behavior now matches that of other operating systems.

Report by Noah Misch

Backpatch through 9.1

contrib/pg_upgrade/util.c

@@ -234,18 +234,15 @@ get_user_info(char **user_name_p)
 /*
  * getErrorText()
  *
- *	Returns the text of the error message for the given error number
- *
- *	This feature is factored into a separate function because it is
- *	system-dependent.
+ *	Returns the text of the most recent error
  */
 const char *
-getErrorText(int errNum)
+getErrorText(void)
 {
 #ifdef WIN32
 	_dosmaperr(GetLastError());
 #endif
-	return pg_strdup(strerror(errNum));
+	return pg_strdup(strerror(errno));
 }