Error Handling for 503 from Airtable

[cyface]

Airtable was momentarily down, and returned a 503:

503 Server Error: Service Temporarily Unavailable for url: https://api.airtable.com/v0/apphAzl2LmaALyWdx/Master%20Page%20List?filterByFormula=%7BWagtail+Page+ID%7D%3D2908

But since that didn't return an actual error response from Airtable, this error message parsing failed:

    code = int(error.split(":", 1)[0].split(" ")[0])
    error_json = error.split("[Error: ")[1].rstrip("]")

with an IndexOutofRange error on the split into error_json in mixins.py line 273.

[KalobTaulien]

@bmoe872's PR was merged. Thanks for reporting and fixing this @cyface and @bmoe872!