Handle request to edit_instrument #4073

In one of our environments the edit_instrument page
was being requested for a contact_link without an
associated instrument. If this should happen notify the user
and redirect the user to the decision page.
Merge pull request #171 from NUBIC/4073-contact_link_instrument_is_nil

Handle request to edit_instrument #4073

The desire to tag 1.6.0.rc3 quickly and send to QA is the reason to merge my own PR.