Improve output for unsupported one-part channel in dotnet-install.sh

When a one-part channel (e.g. `-c 10`) is specified for dotnet-install.sh, the script attempts to resolve it and fails with unclear messaging. The output currently attempts to download files for the one-part channel, leading to a confusing 404 error. The script help text specifies that channels must be 'STS', 'LTS', a two-part version (A.B), or a three-part SDK band (A.B.Cxx), but this expectation isn't reflected in the error message.

Request:
- Detect if a one-part channel is given and update the output to indicate it is not valid.
- Make the error message clear about supported channel formats and provide a suggested workaround (e.g. use `-c 10.0`).

This will help users avoid confusion and parallels a similar situation described in issue #685 (and previously #461).

Repro:
```bash
bash dotnet-install.sh -c 10 --verbose
# output is unclear, attempted downloads fail with 404
```

Expected:
- Clear error about channel format and workaround.

Inizia in dotnet-install.sh tracciando la convalida del channel e il percorso raggiunto da `bash dotnet-install.sh -c 10 --verbose`. Confronta il testo della guida esistente con l’errore risultante e verifica che il channel non supportato composto da una sola parte non provochi più un tentativo di 404. Il lavoro è completato quando l’output spiega i formati supportati e suggerisce di usare un channel composto da due parti, come `10.0`.

Scritto dal modello di indicizzazione a partire dal testo della issue.