re-use tx error type between VM and WASM tx_prelude

[tzemanovic]

follow-up to #2958

Replace the type TxError = String; in namada/src/vm/wasm/run.rs with an error type to be introduced in the core crate and re-used for the error type that's turned to string in macros/src/lib.rs:

                    // TODO: pass some proper error from txs, instead of a string
                    let err = err.to_string().serialize_to_vec();

[sug0]

Also related to #2962