gdal raster hillshade: add error message for unknown ouput format

Author: dbaston

apps/gdaldem_lib.cpp

@@ -3739,6 +3739,8 @@ GDALDatasetH GDALDEMProcessing(const char *pszDest, GDALDatasetH hSrcDataset,
         osFormat = GetOutputDriverForRaster(pszDest);
         if (osFormat.empty())
         {
+            CPLError(CE_Failure, CPLE_AppDefined,
+                     "Could not identify driver for output %s", pszDest);
             return nullptr;
         }
     }

autotest/utilities/test_gdalalg_raster_hillshade.py

@@ -156,6 +156,16 @@ def test_gdalalg_raster_hillshade_vrt_output_pipeline_from_filename():
         alg.Run()
 
 
+def test_gdalalg_raster_hillshade_unknown_format(tmp_vsimem):
+
+    alg = get_alg()
+    alg["input"] = os.path.join(os.getcwd(), "../gdrivers/data/n43.tif")
+    alg["output"] = tmp_vsimem / "out.shp"
+
+    with pytest.raises(RuntimeError, match="Could not identify driver"):
+        alg.Run()
+
+
 def test_gdalalg_raster_hillshade_overview():
 
     src_ds = gdal.Translate(