bad API error message when using a not existing backend

When you use slo-generator in API mode, if you try to compute an SLI with a unknown backend (i mean not specified from the shared configuration), error message from the API is not clear :

Call :
```
curl -X POST -H "Content-Type: text/x-yaml" --data-binary @sli_get-organization_availability.yaml -H "Authorization: Bearer $(gcloud auth print-identity-token)" https://slo-generator-xxxxxxxx-ew.a.run.app
```

Answer:
```
Service Unavailable%
```

Log seen from SLO Generator instance:
```
ERROR - Backend "cloud_monitoring/xxxxxxx" not found in config. Exiting.
```

maybe the error is not well catched ?