Does AES specify error messages when using the wrong password to decrypt? I am writing a small program which uses AES. In testing it with wrong passwords, I get error prompts from Microsoft C# component saying "the padding is bad"; whereas I expect wrongly decoded texts. Do these errors come from the original AES spec or not?
AES in general does not specify that it should return a bad padding message. In fact, AES in general says nothing about padding. Padding schemes are external to AES. Therefore, the message you are getting is .Net specific.

That said, be careful with these messages, as they can lead to a padding oracle attack.