Define and use a base Exception class for the package

[KlaasH]

There are several places where this raises a custom exception with raise Exception(...), e.g.

python-omgeo/omgeo/services/esri.py

Line 91 in 8308301

raise Exception('Must specify both client_id and client_secret to use authentication')

.

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.