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

[ellahathaway]

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

Expected:

• Clear error about channel format and workaround.