Remove client- and app-specific error variants from module errors

[seanchen1991]

Feature Summary

After cleaning up most of the existing String variants from our error types in #1347, there are still a few lingering variants that require some more consideration: ClientError::ClientSpecific and ChannelError::AppSpecific. These two particular variants serve as "backdoors": they allow errors from specific clients (in this case the Tendermint light client) and applications to propagate into the module errors. Ideally, this should not happen.

Proposal

We'll need to come up with a cleaner approach to surfacing client- and application-specific errors, thereby allowing us to remove these backdoor variants from the module errors.