xmlrpc: catch possible exceptions in patch filtering

xmlrpc: catch possible exceptions in patch filtering

Currently, filtering by project, submitter, delegate or state uses a
filter(id=filt[key])[0]. This will throw an exception when something
isn't found, as filter will return [], and getting the first element of
that will fail.

Convert them to explicit get()s, so it's clearer that they can throw an
exception, then catch the 3 possible types of DoesNotExists exceptions.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephenfinucane@hotmail.com>