Fixes #93

Bug: Its possible for the user to open add share window and delete the selected
pool from another browser window. Then if the user add's the share, the backend
throws a cryptic error. Make this error very explicit. Similarly, do this for
the case where all disks from a pool are deleted in some obscure way.
Merge pull request #94 from schakrava/issue#93_pool_filter

Fixes #93