Convert io exceptions to eio

[simonlindholm]

Librcd should use more specific eio exception classes instead of exception_io everywhere. E.g. eof_eio or fs_eio seem particularly useful.

[hnsl]

@simonlindholm When to we get json eio exceptions? Could use it asap. :)

[simonlindholm]

As soon as I manage to compile clang and finish all documentation. :P

[hnsl]

We should do some analyzation and open a series of more specific issues. I have already implemented rio end of stream for example.