Handle parent not found on POST webclient/api/links

[will-moore]

Fixes error reported at https://www.openmicroscopy.org/qa2/qa/feedback/30207/

To test:

• Open the webclient in 2 tabs
• In one, delete a Project
• In the other (without refreshing, drag a Dataset on to the deleted Project)
• Should now see a 404 message instead of a Error.

Error:

Traceback (most recent call last):
...
File "/opt/omero/web/venv3/lib64/python3.6/site-packages/omeroweb/webclient/views.py", line 935, in api_links
return _api_links_POST(conn, json_data)
File "/opt/omero/web/venv3/lib64/python3.6/site-packages/omeroweb/webclient/views.py", line 972, in _api_links_POST
conn.SERVICE_OPTS)
File "/opt/omero/web/venv3/lib64/python3.6/site-packages/omero/gateway/__init__.py", line 4796, in __call__
return self.handle_exception(e, *args, **kwargs)
File "/opt/omero/web/venv3/lib64/python3.6/site-packages/omeroweb/webclient/webclient_gateway.py", line 2133, in handle_exception
e, *args, **kwargs)
File "/opt/omero/web/venv3/lib64/python3.6/site-packages/omero/gateway/__init__.py", line 4793, in __call__
return self.f(*args, **kwargs)
File "/opt/omero/web/venv3/lib64/python3.6/site-packages/omero_api_IQuery_ice.py", line 322, in get
return _M_omero.api.IQuery._op_get.invoke(self, ((klass, id), _ctx))

omero.ValidationException: exception ::omero::ValidationException
{
serverStackTrace = ome.conditions.ValidationException: No row with the given identifier exists: [ome.model.containers.Project#253];

[pwalczysko]

Was able to repeat the problem on outreach server (= without this PR)

On merge-ci, with this PR, the problem is fixed as described.

Ready to merge fmpov