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

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

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/6638c9aaf881b156fafc05371e8ed21f0cdda66e

Modified Files
--------------
contrib/pg_upgrade/file.c |    2 +-
contrib/pg_upgrade/util.c |    2 ++
2 files changed, 3 insertions(+), 1 deletion(-)