admin: Fix error reporting

admin shell erroneously logs remote RuntimeExceptions as bugs. This is
a regression introduced in version 2.9 due to UserAdminShell rethrowing
remote errors.

CellCommandInterpreter already does a local classification of runtime
exceptions into bugs and non-bugs (depending on the throws clause of
the message handler). RuntimeExceptions are propagated and logged
by CellNucleus as uncaught exceptions.

This patch changes the CommandInterpreter to propagate bugs as
CommandPanicException. The admin service is updated to properly
report these as bugs. Likewise, CommandThrowableExceptions are
no longer treated as fatal as these simply represent an regular
error reported by a command. The dispatch code is update to wrap
non-bug remote errors in CommandThrowableException.

Finally, the wording of the bug reporting code in the admin shell
is adjusted for grammar and precision.

Target: trunk
Request: 2.11
Request: 2.10
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Acked-by: Karsten Schwank <karsten.schwank@desy.de>
Patch: https://rb.dcache.org/r/7556/