admin: Propopage NPE to user

I ran into a bug in migration module that caused an NPE to be thrown. Unfortunately
the admin shell return no message at all. This was because the exception was wrapped
in a CacheException with an empty message string.

This patch addresses this problem at several levels: Unexpected exceptions are
now wrapped with message being Throwable#toString (given that we don't know which
except is being wrapped it is reasonable to include the exception type in the error
message), RuntimeExceptions and Errors are propagated unwrapped, and the log output
in admin is increased from warning to error.

Target: trunk
Request: 2.10
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7361/