Define and use a base Exception class for the package

There are several places where this raises a custom exception with `raise Exception(...)`, e.g. https://github.com/azavea/python-omgeo/blob/8308301edf3921b97dd977f8b54b3201a29b3868/omgeo/services/esri.py#L91.

Making users of the package catch `Exception` is not very nice. Even if we don't need subtypes, we should define a base Exception class for the package so that people can make their exception handling more specific.