Adding json load (and syntax) checks in check_steno_config().

I had some namespace conflicts over the "dictionary" variable, so I
explicited both the imported object (SUPPORTED_DICTIONARIES_DICT) and
the assigned object (user_dictionary).
Explicitation of the machine.supported variable.

Since this is a class constant, PEP8 recommends that it should be
written in upper case. Instead of using machine.supported, we're now
using SUPPORTED_MACHINES_DICT, which seems more explicit to me.