Update references to MultiJson to Fog::JSON

Replaces all direct use of `MultiJson` to `Fog::JSON`.
All affected services require `fog/json` via `fog/core`.

`Fog::JSON` will now "slurp" errors raised by `MultiJson` and raise `Fog::JSON::EncodeError` or `Fog::JSON::DecodeError`. These can now be rescued explicitly if desired (there were only 2 cases of this).

Also updates `Fog::Errors::Error` so it will use the error message of the error being "slurped" if the optional message is not given.