Be dynamic about raised exceptions for object not found

[tcmitchell]

pySBOL raises a RuntimeError when an object is not found. The native python SBOL library raises SBOLError. Allow for both by dynamically configuring a NotFoundError like so:

# Set up the not found error for catching
try:
    # SBOLError is in the native python module
    NotFoundError = SBOLError
except NameError:
    # The swig wrapper raises RuntimeError on not found
    NotFoundError = RuntimeError