Looks like the error is just "access denied", presumably because we're trying to open/create the key read/write, where as a normal user process we only typically have read permissions to it?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2884d9bee4a26233c3195f8d05b68aae8277e5c8

commit 2884d9bee4a26233c3195f8d05b68aae8277e5c8
Author: wez <wez@chromium.org>
Date: Wed Feb 15 01:19:26 2017

Log the error result when observing a registry key fails.

Logging a warning on failure isn't very helpful without the error code.

BUG=692259

Review-Url: https://codereview.chromium.org/2693223002
Cr-Commit-Position: refs/heads/master@{#450538}

[modify] https://crrev.com/2884d9bee4a26233c3195f8d05b68aae8277e5c8/chrome/browser/extensions/external_registry_loader_win.cc

@comment #1, we use KEY_NOTIFY flag, which seems to be treated as write?
Regardless, as you suggested in the CL, we should be reading the registry key once during startup if watching the key fails.
I thought we already had code to read the registry once during startup. If so,  we don't need to do anything else and continue. If not, then I'll add it.
Assigning to myself.