Handle missing model in CLI parameters for llama-run

[engelmi]

The HTTP client in llama-run only prints an error in case the download of a resource failed. If the model name in the CLI parameter list is missing, this causes the application to crash.
In order to prevent this, a check for the required model parameter has been added and errors for resource downloads get propagated to the caller.

This could cause llama-run to crash:

$ llama-run --jinja
curl_easy_perform() failed: HTTP response code said error
terminate called after throwing an instance of 'nlohmann::json_abi_v3_11_3::detail::parse_error'
  what():  [json.exception.parse_error.101] parse error at line 1, column 1: attempting to parse an empty input; check that your input string or stream contains the expected JSON
Aborted (core dumped)

With the model name missing, it proceeds to try and download the resource. The curl error gets printed, but isn't handled further and so the json parsing crashes the program.

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[engelmi]

@ericcurtin PTAL