Define an error type that captures all error variants shared between ibc-rs's errors

[seanchen1991]

This will serve to streamline and de-duplicate error variants shared amongst ibc-rs's different error types. @rnbguy suggestion is as follows:

pub enum GenericError<HostError> {
    HostError(HostError),
    DecodeError(..),
    EncodeError(..),
    ...
}

For example, any error related to decoding/encoding from any module would point to the GenericError::DecodeError variant. This should standardize error logging a bit, as well remove lots of extraneous variants that are copied between ibc-rs's different error types.

[seanchen1991]

Closed in favor of #1319