When a Node fails to start, it will throw a StartupException on the
main thread which will cause the process to exit.
Previously these were simply logged in the same way as any other
uncaught exception, which would potentially result in long stack
traces with the key details (the primary cause) being nested somewhere
in the middle of the log lines. This was particularly true if the
failure was due to an exception being thrown within a plugin - the
primary cause may well have been wrapped in two or three other
exceptions before it was logged.

This commit adds a new summarised description whenever there is an
uncaught StartupException. This summary is logged before and after the
standard stack trace logging to make it more prominent and increase
the likelihood that it will be noticed and understood.

The summary focuses on printing messages from ElasticsearchExceptions
as these are the most likely to hold clear, specific and actionable
information and also prints the message for each cause of the
ElasticsearchException which may contain the precise details (e.g. the
pathname in a FileNotFoundException or AccessDeniedException).

Resolves: elastic#34895