Part of https://github.com/elastic/kibana-team/issues/646
This PR adds an example plugin in `examples/error_boundary` that shows
usage of KibanaErrorBoundary.
The example plugin is used in a functional test to ensure errors are
caught in the appropriate way, and error messages include a working
Refresh button.
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Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
