oiiotool silently extracts thumbnail instead of raw image

### Problem

If oiio is built without libraw attempting to read a raw file will silently extract the embedded thumbnail instead of the raw data. This behavior was observed using oiiotool from a Windows build of 1.8.5 to read Canon .cr2 files.

You can tell that the thumbnail is being output because the resulting image is a different sized than it is when extracting the raw data using a libraw enabled build. Passing options such as use_camera_wb also do not affect the output as the white balance is baked in to the thumbnail.

**Expected behavior:**

Extracting the thumbnail could be useful, but is probably not the desired behavior in most cases. If OIIO can't read the raw data because libraw isn't present the user should be warned.

**Actual behavior:**

Currently, oiiotool provides no indication of what has happened, even if run with the -v or --debug options.

Start with oiiotool's handling of Canon .cr2/raw files in a build without libraw, comparing it with a libraw-enabled build. Check the -v and --debug paths; done when a missing-libraw read no longer silently returns the embedded thumbnail and clearly warns the user.

Written by the indexing model from the issue text.