needextend: new link to not existing needs crashes

[danwos]

If I set an extra-link via needextend like this:

.. needextend:: NEED_1
   :own_link_type: NOT_EXISTING_NEED_ID

needextend.py throws an error:

  File "[...]/python3.8/site-packages/sphinx_needs/directives/need.py", line 393, in process_need_nodes
    process_needextend(app, doctree, fromdocname)
  File "[...]/python3.8/site-packages/sphinx_needs/directives/needextend.py", line 142, in process_needextend
    if found_need["id"] not in env.needs_all_needs[ref_need][f"{option_name}_back"]:
KeyError: 'NOT_EXISTING_NEED_ID'

The reason is that this code:
found_need["id"] not in env.needs_all_needs[ref_need][f"{option_name}_back"]
is using env.needs_all_needs[ref_need] and ref_need is referencing a not existing need.

Solution: Check if the link target exists in advance.