Runtime client should retry checking blocks

[ptrus]

Runtime client should periodically re-check blocks in case checking block for transaction fails (e.g. due to a no storage node being available):

oasis-core/go/runtime/client/watcher.go

Lines 69 to 73 in 1e3a6fe

	matches, err := tree.GetTransactionMultiple(ctx, txHashes)
	if err != nil {
	w.Logger.Error("can't get block I/O from storage", "err", err)
	return
	}

Discovered in the daily e2e tests.

[kostko]

This is similar to #3388.