Replace RuntimeError Exceptions with Subclassed Exceptions

[j9ac9k]

Currently, when the registration objects are initialized, it checks to see if the GeoData objects have had their data "prepped". If not, it raises a RuntimeError, with a message about ensuring the prep method on the GeoData object was run.

As other code in the stack can also raise RuntimeError's, we should likely create our own exception that users can specifically try and catch.