tsmt: export smt error

src/merkle/mod.rs

@@ -27,7 +27,7 @@ mod simple_smt;
 pub use simple_smt::SimpleSmt;
 
 mod tiered_smt;
-pub use tiered_smt::{TieredSmt, TieredSmtProof};
+pub use tiered_smt::{TieredSmt, TieredSmtProof, TieredSmtProofError};
 
 mod mmr;
 pub use mmr::{Mmr, MmrPeaks, MmrProof};