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

Bruce Momjian <bruce@momjian.us>

Commit: f91c4e326aa9c81223146b43b4167c29630155c4
Author: Bruce Momjian <bruce@momjian.us>
Date: 2015-11-24T22:18:28Z
Releases: 9.4.6
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

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