Prevent zpool upgrade failing due to unavailable pools

Prior to this fix "zpool upgrade" and "zpool upgrade -a" would fail due to
an assert when operating on unavailable pools.

We now print a warning to stderr but allow the processing of other pools
to procesed.

MFC after:	1 month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276194 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f