e2e: Replace `AssertValidTxResponse` and error check with `AssertSuccessTx` or `AssertFailedTx`

[colin-axner]

Summary

Instead of asserting that a response from broadcasting a tx is not nil, we should immediately fail if it is nil and provide helper functions to see if the tx was successful or not. See comment:

Instead of:

    transferTxResp, err := s.Transfer(args)
    s.Require().NoError(err)
    s.AssertValidTxResponse(transferTxResp)

use

    resp := s.Transfer(args)
    s.AssertTxSuccess(resp)

or

    resp := s.Transfer(args)
    s.AssertTxFailure(resp, transfertypes.ErrInsufficientBalance) // expected error code 

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[damiannolan]

I like the idea of AssertTxSuccess and AssertTxFailure. Is the idea here to also remove the error return from BroadcastMessages? E.g. replacing this code block with s.Require().NoError(err)

[chatton]

Updated link to BroadcastMessages and the error return