Simplify update error message support HTML.

Simplify update error message support HTML.

Errors from the update mechanism were appearing twice, once
included in the error string generated in google_update_win
and once in OnError() of version_updater_win.  This caused
redundant information like this:

Update failed (error: 1)An error occurred while checking for updates: Update check failed to start (error code 1: 0x80004005).

Now this:

An error occurred while checking for updates: Update check failed to start (error code 1: 0x80004005).

In addition, this allows removal of the check against a specific
error code since that comes with an actual error message already.

The error message has been explicitly stated as being HTML, making
it okay for callers to include mark-up such as links to more info.

For more information, see the discussion on this CL:
https://codereview.chromium.org/1212783003/

BUG=489801

Review URL: https://codereview.chromium.org/1214603005

Cr-Commit-Position: refs/heads/master@{#338282}