Exceptions handling and structure

1. We should revisit the whole codebase to ensure exceptions are well defined and handled well.
2. We don't have a good picture of the exception tree now. As a reference, in trinity, the package trinity has its own exception tree and so do p2p. I'm not sure what is the best structure for us, but I think probably a single exception tree(i.e. all exceptions inherit from BaseLibp2pError and its descendants) can be a start?

@ChihChengLiang @ralexstokes @NIC619 @pipermerriam

Would love to have your comment on it:)