Handle error when adding pending contact

Related MR:
3 jobs for 76-adding-contact-errors in 5 minutes (queued for 5 seconds)
Name Stage Failure
pytest Test
ERROR briar-gtk/tests/briar_gtk/test_window.py::test_startup_container_at_init
ERROR briar-gtk/tests/briar_gtk/test_window.py::test_window_actions_at_init
ERROR briar-gtk/tests/briar_gtk/test_window.py::test_window_add_at_init - Att...
ERROR briar-gtk/tests/briar_gtk/test_window.py::test_show_main_container - At...
ERROR briar-gtk/tests/briar_gtk/test_window.py::test_show_main_shown - Attrib...
ERROR briar-gtk/tests/briar_gtk/test_window.py::test_show_main_add - Attribut...
ERROR briar-gtk/tests/briar_gtk/test_window.py::test_show_main_destroy_old - ...
========================= 15 passed, 8 errors in 2.88s =========================
ERROR: Job failed: exit code 1
pylint Test
briar-gtk/briar_gtk/add_contact/add_contact_view.py:163:8: C0103: Variable name "e" doesn't conform to snake_case naming style (invalid-name)
briar-gtk/briar_gtk/add_contact/add_contact_view.py:165:8: C0103: Variable name "e" doesn't conform to snake_case naming style (invalid-name)
briar-gtk/briar_gtk/add_contact/add_contact_view.py:203:4: R0913: Too many arguments (6/5) (too-many-arguments)
briar-gtk/briar_gtk/add_contact/add_contact_view.py:18:0: C0412: Imports from package briar_wrapper are not grouped (ungrouped-imports)

Your code has been rated at 9.96/10

ERROR: Job failed: exit code 1
pycodestyle Test
    E701: else: do_non_blah_thing()
E701: try: something()
E701: finally: cleanup()
E701: if foo == 'blah': one(); two(); three()
E702: do_one(); do_two(); do_three()
E703: do_four(); # useless semicolon
E704: def f(x): return 2*x
E731: f = lambda x: 2*x
ERROR: Job failed: exit code 1