Better exceptions

[MrNaif2018]

Currently, upon any error got from server(json data's error key), it is just raised like
raise ValueError(json["data"])
There should be more error classes, maybe autogenerated. Or at least there should be a way to differentiate errors. (instead of doing like if "Error loading wallet" in str(e))
Also, as we have bitcart and bitcart-async, there might be a need in creating special ConnectionError because exceptions in async and sync version differ.

[MrNaif2018]

Done in #15