Fix exception when parts of Pollen.com can't be reached

[bachya]

Description:

#15286 revealed an unhandled exception when parts of Pollen.com's back-of-house API are down. This PR halts updating when such an exception occurs.

Related issue (if applicable): fixes #15286

Pull request in home-assistant.github.io with documentation (if applicable): N/A

Example entry for configuration.yaml (if applicable):

sensor:
  - platform: pollen
    zip_code: "12345"
    monitored_conditions:
      - allergy_average_forecasted
      - allergy_average_historical
      - allergy_index_today
      - allergy_index_tomorrow
      - allergy_index_yesterday
      - disease_average_forecasted

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass

If user exposed functionality or configuration variables are added/changed:
- [ ] Documentation added/updated in home-assistant.github.io

If the code communicates with devices, web services, or third-party tools:
- [ ] New dependencies have been added to the REQUIREMENTS variable (example).
- [ ] New dependencies are only imported inside functions that use them (example).
- [ ] New or updated dependencies have been added to requirements_all.txt by running script/gen_requirements_all.py.
- [ ] New files were added to .coveragerc.

If the code does not interact with devices:
- [ ] Tests have been added to verify that the new code works.