startup: fix logging multiple stacktraces on RuntimeException

When dCache fails to start up a service because a cell throws
a RuntimeException then the same stack-trace is logged three times:
once by AbstractCell's doInit and twice within Bootload.

This patch reduces this to just once.  It also fixes the problem
that multiple RuntimeExceptions have null as their message, so
toString is used instead.

Note that there is still a RuntimeException that wraps the previous
RuntimeException and adds the context information (which line and file
triggered the problem), therefore the stack-trace includes two
exceptions.

Target: master
Requires-book: no
Requires-notes: no
Patch: http://rb.dcache.org/r/5919
Acked-by: Tigran Mkrtchyan