Fix two more errors with missing scan objects

In commit c38ecb0, an error was fixed whereby an attempt was be made to
determine whether a scan existed for a particular vendor using the
following condition:

    if vendor.scan != None:

Due to the current (soon-to-be-replaced) Navigator implementation, this
condition is (always?) true; vendor.scan is a Navigator object which has
been created with lazy-fetching, but has not yet retrieved any of the
values. When an attempt is subeqently made to access one of the
properties, and the scan did not actually exist on the server, this
would cause an exception to be thrown. Thus, in addition to checking
this condition, you also have to call refresh_resource() to find out if
it exists.

The two specific cases fixed here, in startScan and pruneScan, were
causing problems with hapi-worker. In startScan, it would think there
was already a scan running, and thus never actually start one. In
pruneScan, it would think a scan was present when it sometimes actually
wasn't, and encounter an exception when trying to delete the object
(thus causing 'hapi-worker killscans' to fail).