MPDC: Throw exceptions instead of Either

- Future always return with a Try, which makes the Either more or less obsolete
- Cleans up a lot of code
- web will be fixed in next commit
Merge branch 'master' of https://github.com/ekroth/mpd-monad
Fix web MPDC usage according to previous commit

- Minor code clean up
- Use new MPDC system
- Still need to add error checking