Add simple NoDataAvailableError exception for missing data handling

- Introduce a lightweight `NoDataAvailableError` exception to handle
  cases where no data is available for analysis or plotting.
- Add a check after data retrieval in `run_workflow()` to raise a
  `NoDataAvailableError` if reporting data is empty. This prevents
  proceeding with empty datasets and ensures graceful failure.

Signed-off-by: cyiallou - Costas <[email protected]>

Author: cyiallou

src/frequenz/lib/notebooks/solar/maintenance/solar_maintenance_app.py

@@ -119,6 +119,10 @@ class SolarAnalysisData:
     )
 
 
+class NoDataAvailableError(Exception):
+    """Raised when there is no available data."""
+
+
 # pylint: disable=too-many-statements, too-many-branches, too-many-locals
 async def run_workflow(user_config_changes: dict[str, Any]) -> SolarAnalysisData:
     """Run the Solar Maintenance App workflow.
@@ -142,6 +146,7 @@ async def run_workflow(user_config_changes: dict[str, Any]) -> SolarAnalysisData
                 `stat_profile_view_col_to_plot`) are hardcoded.
             - If the timezone of the data does not match the timezone in the
                 configuration.
+        NoDataAvailableError: If no reporting data is available.
     """
     config, all_client_site_info = _load_and_validate_config(user_config_changes)
 
@@ -193,6 +198,9 @@ async def run_workflow(user_config_changes: dict[str, Any]) -> SolarAnalysisData
         )
     reporting_data_higher_fs = await retrieve_data(reporting_config)
 
+    if reporting_data.empty and reporting_data_higher_fs.empty:
+        raise NoDataAvailableError("No reporting data available. Cannot proceed.")
+
     weather_data = transform_weather_data(
         data=weather_data,
         weather_feature_names_mapping=config.weather_feature_names_mapping,