Properly handle Google API service errors

[kylegibson]

Sometimes the google api may not be available. In that case, we need to be able to retry in a few minutes

Traceback (most recent call last):
  File "/var/task/flask/app.py", line 2292, in wsgi_app
  response = self.full_dispatch_request()
  File "/var/task/flask/app.py", line 1815, in full_dispatch_request
  rv = self.handle_user_exception(e)
  File "/var/task/flask/app.py", line 1718, in handle_user_exception
  reraise(exc_type, exc_value, tb)
  File "/var/task/flask/_compat.py", line 35, in reraise
  raise value
  File "/var/task/flask/app.py", line 1813, in full_dispatch_request
  rv = self.dispatch_request()
  File "/var/task/flask/app.py", line 1799, in dispatch_request
  return self.view_functions[rule.endpoint](**req.view_args)
  File "/var/task/main.py", line 77, in handle_incoming_github_event
  handle_merged_pr(event)
  File "/var/task/main.py", line 161, in handle_merged_pr
  update_spreadsheet(book, event)
  File "/var/task/main.py", line 239, in update_spreadsheet
  display_name = get_gh_login_display_name_mapping(book, merged_by)
  File "/var/task/main.py", line 253, in get_gh_login_display_name_mapping
  sheet = book.worksheet(DEV_DASHBOARD_GH_LOGIN_LOOKUP_SHEET_NAME)
  File "/var/task/gspread/models.py", line 235, in worksheet
  sheet_data = self.fetch_sheet_metadata()
  File "/var/task/gspread/models.py", line 184, in fetch_sheet_metadata
  r = self.client.request('get', url, params=params)
  File "/var/task/gspread/client.py", line 79, in request
  raise APIError(response)
gspread.exceptions.APIError: {
  "error": {
  "code": 503,
  "message": "The service is currently unavailable.",
  "status": "UNAVAILABLE"
  }
}