Don't hide exception context behind ExceptionType

[PascalTurbo]

For example when trying to create a document with nil key, an exception will be raised:

connectlocal.rb:7:in `<main>': request not scheduled for  (Libcouchbase::Error::EmptyKey)

When handling errors you would do something like this:

begin
    couchbase.add(nil, 123)
rescue StandardError => e
    puts e
    puts e.backtrace if DEBUG
end

This will lead to the useless information request not scheduled for because the information about the problem is only present in the error class: Libcouchbase::Error::EmptyKey