Improve exception-handling

[igrep]

1. Make a policy for exception-handling:
   • Public APIs return explicitly IO (Either Exception a). To make them handlable by users and alert users by their documents.
   • Internal, and automatically called APIs throwIO-s. Because they are a bit hard to handle, and they should usually be successful without exceptions.
2. Add methodName field to UnexpectedReponse data constructor
   • To clarify which RPC calling throws an exception.
   • Would help detecting the cause without stack traces.
3. Refacor the verbose void $ rethrowingException-s with the new callRpcThrow function.
4. Avoid incomplete pattern-matches to throw the errors as Web.Direct.Exception.Exception as much as possible.

[kazu-yamamoto]

Merged.