Werk #16799: Custom & forecast graphs: More descriptive error messages in case of missing user input when configuring metrics

When configuring the metrics rendered in custom and forecast graphs, users have to select a host, a service and a metric. Previously, if any of these fields were missing, the Checkmk UI displayed the message "Cannot calculate graph recipes" and an uninformative traceback. As of this werk, the UI instead displays a descriptive error message.

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