message: Better error message for obsolete MessageTooLong return code
diff: @@ -279,7 +279,7 @@ impl Api for ExternalApi {
  match result {
  0 => Ok(true),
  1 => Ok(false),
  - 2 => panic!("MessageTooLong must not happen. This is a bug in the VM."),
  + 2 => panic!("Error code 2 unused since CosmWasm 0.15. This is a bug in the VM."),
  3 => panic!("InvalidHashFormat must not happen. This is a bug in the VM."),
  4 => Err(VerificationError::InvalidSignatureFormat),
  5 => Err(VerificationError::InvalidPubkeyFormat),
  @@ -311,7 +311,7 @@ impl Api for ExternalApi {
  match result {
  0 => Ok(true),
  1 => Ok(false),
  - 2 => panic!("MessageTooLong must not happen. This is a bug in the VM."),
  + 2 => panic!("Error code 2 unused since CosmWasm 0.15. This is a bug in the VM."),
  3 => panic!("InvalidHashFormat must not happen. This is a bug in the VM."),
  4 => Err(VerificationError::InvalidSignatureFormat),
  5 => Err(VerificationError::InvalidPubkeyFormat),
  