Improve differing sector size error

When adding or replacing a vdev with a different sector size the
error message should be more useful.  In addition to describing
the problem provide a hint that the '-o ashift' option can be
used to override the optimal default value.

Since using a non-optimal value may incur a significant performance
penalty we should issue this error.  But there a numerous reasons
why a administrator may wish to do this anyway.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2421