Merge PacketError type into ChannelError

[seanchen1991]

Feature Summary

ibc-rs's top-level error type contains two distinct ICS04 variants, ChannelError and PacketError. This is a discrepancy from the rest of the variants that exist in this error type, as every other variant captures all the errors that originate from a single module. This discrepancy should be cleaned up by merging ChannelError and PacketError into a single type.

Proposal

Merge the PacketError type into ChannelError by moving the relevant PacketError variants into ChannelError. We'll opt to do this instead of simply adding an Packet(PacketError) variant under ChannelError in order to avoid unnecessary enum nesting.