SPA load/import should tell us why it failed
When we fail to open or import a storage pool, we typically don't get any
additional diagnostic information, just "no pool found" or "can not import".
While there may be no additional user-consumable information, we should at
least make this situation easier to debug/diagnose for developers and support.
For example, we could start by using `zfs_dbgmsg()` to log each thing that we
try when importing, and which things failed. E.g. "tried uberblock of txg X
from label Y of device Z". Also, we could log each of the stages that we go
through in `spa_load_impl()`.