MFV r271225:

Iterate through all the children instead of returning error when we hit
the first error.  This makes the error message give more information
rather than just the first device that causes problem.

Illumos issue:
    5118 When verifying or creating a storage pool, error messages only
	 show one device

MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271227 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f