Fix zpool status -v error message

When a pool is suspended it's impossible to read the list
of damaged files from disk. This would result in a generic
misleading "insufficient permissions" error message.

Update zpool_get_errlog() to use the standard zpool error
logging functions to generate a useful error message. In
this case:

  errors: List of errors unavailable: pool I/O is currently suspended

This patch does not address the related issue of potentially
not being able to resume a suspend pool when the underlying
device names have changed.

Additionally, remove the error handling from zfs_alloc()
in zpool_get_errlog() for readability since this function
can never fail.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4031
Closes #5731
Closes #5907