usb: Use Mojo union to separate device open success from error codes

Mojo doesn't support a void type (issue 1220106) but we can emulate one
with an enum that has only one defined value. This change adds a new
UsbOpenDeviceResult union which is either a UsbOpenDeviceSuccess or a
UsbOpenDeviceError.

Change-Id: If158c1dd52315fd1bf51062a4b8e64a3ed967a84
Bug: None
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4234077
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1105463}