Improve IT2Me error logging.

Improve IT2Me error logging.

When a host is unable to set-up sharing for IT2Me, there is currently no
reportable error message (with the exception of failure due to domain
policy). This CL adds an error message to the native messaging protocol,
logging it to the console. In the longer term, as we identify common
failures, it makes sense to treat them like we do kInvalidDomainError,
and show a proper error message, but for now this will help us track
down what the common failures are.

As part of cleaning this up, I've deprecated web-app support for errors
being reported as a host state change message. These were being handled
the same way as the top-level "error" message, and the latter has the
advantage of not being tied to a host state change.

BUG=505627

Review URL: https://codereview.chromium.org/1216743005

Cr-Commit-Position: refs/heads/master@{#336706}