Bug 30057: Move virtualshelf-related exceptions to a separate library

Bug 30057: Move virtualshelf-related exceptions to a separate library

This patch moves the not-generic virtualshelf-related exceptions to
their own file. Callers are adjusted as required.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/Virtualshelves.t
=> SUCCESS: Tests pass
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests pass
4. Check virtualshelves work as expected
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>