fix: give a better error on attempting to run raster with no sample

src/aare/daq/daq.py

@@ -1275,6 +1275,8 @@ class AareDAQ:
         Returns:
             CompletedRasterGrid result or None if failed
         """
+        if self.sample is None:
+            raise RasterScanException("Please load a sample before running a raster scan. If you have manually mounted a sample, create its record in the 'Manual sample' panel.")
         try:
             logger.info(
                 "Starting raster sequence",
@@ -2037,7 +2039,7 @@ class AareDAQ:
             result = self._execute_raster_sequence(request, auto_center=auto_center)
 
             if result is None:
-                raise RasterScanException("Raster scan failed")
+                raise RasterScanException("Raster scan failed for unknown reason.")
             self._set_state(BeamlineStateEnum.SampleAlignment)
             return result
         except Exception: