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
poolmanager: provide reasonable error message if partition type is unknown

If the admin has saved a partition type 'classic' in their
poolmanager.conf then dCache will fail to start with a meaningless
error message and a stack-trace.

This patch provides a meaningful error message.  Due to a
hack/work-around for another problem, the message is wrapped in a
RuntimeException, which triggers reporting the problem as a bug (with
a stack-trace).

Target: master
Requires-notes: no
Requires-book: no
Patch: http://rb.dcache.org/r/5920/
Acked-by: Karsten Schwank
pom: yet another fix to datanucleus dependency

use the same plugin across components

Acked-by: Paul Millar
Acked-by: Karsten Schwank
Target: master
Require-book: no
Require-notes: no
Patch: http://rb.dcache.org/r/5923/
[maven-release-plugin] prepare branch 2.7
[maven-release-plugin] prepare for next development iteration