Unable to save a child if the postcode validation server is down

[mbarton]

1. Edit POSTCODE_API_BASE_URL to point to something else, to simulate the postcode validation API being down
2. Try to add a new child

Expected: the child saves fine
Actual: it fails to save saying the postcode is invalid

django-1   | ERROR [epilepsy12.general_functions.postcode:31] Postcode validation failure. Could not validate postcode at https://mrb.lol/postcodes/SE12 9QF. response.status_code=404
django-1   | INFO [epilepsy12.views.case_views:624] Invalid data provided to case form: <ul class="errorlist"><li>postcode<ul class="errorlist"><li>Invalid postcode</li></ul></li></ul>

May be worth fixing alongside moving to our own hosted postcodes.io (#1096)