API: raise `OSError` when a GraphViz executable is not found
API: raise `OSError` when a GraphViz executable is not found

Python 3 instantiates a subclass of `OSError` that is more
specific to the kind of error raised [1].

By raising an error more specific than `Exception`,
it can be recognized by packages that use `pydot`.

[1] https://docs.python.org/3/library/exceptions.html#OSError
[2] https://docs.python.org/3/library/exceptions.html#FileNotFoundError

pydot/pydot,erocarrera/pydot