sig_generate: `listfilteredmutations` should raise error if API query fails

[gordonkoehn]

This function does not properly implement Exceptions, but prints to the console:

(not using access keys on API https://lapis.cov-spectrum.org/open/v2)
Error from server: {
  "error": {
    "type": "about:blank",
    "title": "Bad request",
    "status": 400,
    "detail": "Error from SILO: The lineage 'BA.5* | nextcladePangoLineage:BA.5*' is not a valid lineage for column 'nextcladePangoLineage'."
  },
  "info": {
    "dataVersion": null,
    "requestId": "c58424fb-2152-4aaf-97f3-157ee51bcd57",
    "requestInfo": "sars_cov-2_nextstrain_open on lapis.cov-spectrum.org at 2025-04-17T12:35:10.523943395",
    "reportTo": "Please report to https://github.com/GenSpectrum/LAPIS/issues in case you encounter any unexpected issues. Please include the request ID and the requestInfo in your report.",
    "lapisVersion": "0.4.2",
    "siloVersion": "0.5.8"
  }
}

This issue is only important should we decide to use CoJac for querying CovSpectrum.

If this remains a script utility it is perfectly fine as is.

[gordonkoehn]

This this is the root cause - better practice would be try/catch as listfilteredmutations will yield null otherwise.

def checkerror(reply):
    if reply.get("data", None) is None:
        # TODO prety-printing would help
        print("Error from server:", json.dumps(reply, indent=2))
        # TODO replace with proper exception throwing in the future
        sys.exit(1)

    return reply