message: [types] use From instead of TryFrom for SignedTransaction
  In order to preserve the same semantics we have with the
  FromProto/IntoProto traits, lets convert the TryFrom impl for
  SignedTransaction to an infallible From impl.
  Closes:
  Approved by: andll
diff: @@ -534,12 +534,11 @@ impl TryFrom<crate::proto::types::SignedTransaction> for SignedTransaction {
  }
  }
  -impl TryFrom<SignedTransaction> for crate::proto::types::SignedTransaction {
  - type Error = Error;
  -
  - fn try_from(txn: SignedTransaction) -> Result<Self> {
  - let signed_txn = SimpleSerializer::<Vec<u8>>::serialize(&txn)?;
  - Ok(Self { signed_txn })
  +impl From<SignedTransaction> for crate::proto::types::SignedTransaction {
  + fn from(txn: SignedTransaction) -> Self {
  + let signed_txn = SimpleSerializer::<Vec<u8>>::serialize(&txn)
  + .expect("Unable to serialize SignedTransaction");
  + Self { signed_txn }
  }
  }
  