Handle failure from image converter service or other HttpErrors in `wget()`

If `wget()` in PortalBase `(https://github.com/adafruit/Adafruit_CircuitPython_PortalBase/blob/7a3277af2efb804c7ecf82d8bc87eb95c22dee2e/adafruit_portalbase/network.py#L319) gets some kind of HTTP error, it will raise `adafruit_portalbase.network.HttpError`, which is a subclass of `Exception`.

This exception is not handled when `wget()` is called here:
https://github.com/adafruit/Adafruit_CircuitPython_PyPortal/blob/a14d41bff67ec9411e8c7b5b9b8425ff39dae807/adafruit_pyportal/network.py#L191-L196.

A scenario where this happened is described in the forums [here](https://forums.adafruit.com/viewtopic.php?t=204689): the Cleveland Museum of Art may return image URL's that return 404's. These cause the image converter service to return a 422.

I'm thinking that maybe PortalBase should return an easier-to-catch exception, or PyPortal should catch that HttpError and turn it into something else.

Start with adafruit_portalbase/network.py around wget() at line 319, then inspect adafruit_pyportal/network.py around lines 191-196 where it is called. Trace the HttpError raised for HTTP failures such as the described 404-to-422 case and decide which layer should handle or translate it. Done means the converter failure no longer escapes as an unhandled exception.

Written by the indexing model from the issue text.