The error
gpg: decryption failed: Bad session key
is only logged if the sanity check "algo given in decrypted session key is a valid OpenPGP algo" passes even though a wrong password was given (which happens with a chance of 11:256). If the sanity check detects a bad algo then gpg logs
gpg: decryption of the symmetrically encrypted session key failed: Checksum error
If AEAD is used, then other logging will happen.