fix: show error message when decode tx fails

[pedroferreira1]

Context

#331

The decode-tx API does not return success: False when the decoding fails, that's why the current code has a bug.

Acceptance Criteria

• The error message should be correctly shown when decoding an invalid tx.

Security Checklist

• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.