Remove error string from transfer. Replace with constant string

[colin-axner]

Summary

Any values included in an acknowledgement will be committed in the state machine. Thus the error message returned from OnRecvPacket is state machine breaking if it changes. This makes our code hard to change in patch releases (and this restriction applies to our dependencies). We should remove the err.Error() from the transfer ack and replace with a constant error string that won't change

see #744 for more information

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[colin-axner]

The abci error code can be emitted in the error acknowledgement.

[damiannolan]

The abci error code can be emitted in the error acknowledgement.

This refers to just ics27, right? Transfer just requires a string const here.

NewErrorAcknowledgement() in 04-channel/types should probably be documented to specify a deterministic error string is required. Even still, we can't guarantee that developers will provide that.

[colin-axner]

This refers to just ics27, right? Transfer just requires a string const here.

That string const being deterministic rests on the assumption that error strings are included in the state machine, which is not true. The ABCI code could be included in the same way it was done for ICS27 in #794