Restore old pgwin32_message_to_UTF16() behavior outside transactions.

Commit 49c817ea replaced with a hard
error the dubious pg_do_encoding_conversion() behavior when outside a
transaction.  Reintroduce the historic soft failure locally within
pgwin32_message_to_UTF16().  This fixes errors when writing messages in
less-common encodings to the Windows event log or console.  Back-patch
to 9.4, where the aforementioned commit first appeared.

Per bug #13427 from Dmitri Bourlatchkov.