Improved documentation for aggregate_polygon. (#29)

aggregate_polygon.json

@@ -1,7 +1,7 @@
 {
     "id": "aggregate_polygon",
     "summary": "Compute zonal statistics for polygons",
-    "description": "Aggregates zonal statistics for one or multiple polygons over the spatial dimensions.\n\nThe number of total and valid pixels is returned together with the calculated values.",
+    "description": "Aggregates zonal statistics for one or multiple polygons over the spatial dimensions.\n\nThe data cube must have been reduced to only contain two raster dimensions and a third dimension the values are aggregated from, for example a single band. Otherwise this process fails with the `TooManyDimensions` error.\n\nThe number of total and valid pixels is returned together with the calculated values.",
     "categories": [
         "cubes",
         "aggregate"
@@ -73,6 +73,11 @@
             ]
         }
     },
+    "exceptions": {
+        "TooManyDimensions": {
+            "message": "The dimensions need to be reduced to three for 'aggregate_polygon'."
+        }
+    },
     "links": [
         {
             "href": "https://github.com/Open-EO/openeo-processes/issues/2",