Handle error from m0_layout_find_by_objsz()

[andriytk]

Such error can happen, for example, when there are too many
failures in the cluster which are not tolerable by motr, so
the pool version is not available and m0_layout_find_by_objsz()
cannot return any meaningful result.

Solution: handle errors from m0_layout_find_by_objsz().

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[andriytk]

See also Seagate/cortx-motr#1838