Don't rely on HRESULTs to detect locked bookmark files

[nblumhardt]

The durable log shipper checks the HRESULT behind an IOException in order to detect locked log files.

https://github.com/serilog/serilog-sinks-seq/blob/dev/src/Serilog.Sinks.Seq/Sinks/Seq/HttpLogShipper.cs#L397

Since this is exposed in .NET Core we didn't pick it up in the migration, but obviously this won't work on non-Windows platforms (https://github.com/dotnet/corefx/issues/11144).

It's probably safe to assume on .NET Core that any IOException here indicates a locked file, we're only doing the check to improve diagnostics.

Thanks @ghuntley for raising this 👍