internal/keyspan: fix InterleavingIter error handling

Previously, the InterleavingIter could violate the InternalIterator contract
when an error is encountered. If one of its two child iterators encountered an
error and returned a nil KV pair, the iterator could yield a non-nil KV to the
caller. This commit updates the interleaving iterator error handling to
accumulate any encountered errors on a field and always yield nil.

This bug was surfaced by a new 'invariants' tag assertion in Valid, asserting
that Error() returns nil if the iterator is valid. With this invariants tag,
existing tests surface the assertion failure. Additional test coverage will
come with #1115.