Don't give up if a bundle isn't available.

(This is temporarily moot because the 8.x bundles are no longer being downloaded, but will come up again when two versions are available.)

(I used the wrong library name in this example, but that doesn't matter.)

```
$ circup install adafruit_sht31
Version 2.1.1 of circup is outdated. Version 2.1.2 was released 2 days ago.
Found device metro_m4_airlift_lite at /media/halbert/CIRCUITPY, running CircuitPython 9.2.3.
A newer version of CircuitPython (9.2.4) is available.
Get it here: https://circuitpython.org/board/metro_m4_airlift_lite
Downloading latest bundles for adafruit/Adafruit_CircuitPython_Bundle (20250214).
py:
Extracting: [####################################] 100%
8.x-mpy:
There was a problem downloading that platform bundle. Skipping and using existing download if available.
Downloading latest bundles for adafruit/CircuitPython_Community_Bundle (20250216).
py:
Extracting: [####################################] 100%
8.x-mpy:
There was a problem downloading that platform bundle. Skipping and using existing download if available.
Searching for dependencies for: ['adafruit_sht31']
WARNING:
adafruit_sht31 is not a known CircuitPython library.
Ready to install: []
```
It should go ahead and try to download the 9.x-mpy bundles as well, but it didn't.

Start by reproducing the circup install command shown in the issue and trace the bundle download fallback when the 8.x-mpy bundle is unavailable. The work is done when circup also attempts the 9.x-mpy bundles and can continue resolving the requested library when those bundles are available.

Written by the indexing model from the issue text.