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Show any connection errors during startup in the LoadingWindow rather
than creating a new error window for the error.

This cl also adds a new updateErrorMessage method to the MessageWindow
to update the message and reset the message window as appropriate for
an error message (disabling the spinner, updating the button label).

BUG=

Review URL: https://codereview.chromium.org/1139543002

Cr-Commit-Position: refs/heads/master@{#330022}