return error code when no feature can be read from shapefile + update unit test.

test/unit/datasource/shapeindex.cpp

@@ -162,9 +162,13 @@ TEST_CASE("shapeindex")
                         // count features
                         std::size_t feature_count = count_shapefile_features(path);
                         // create *.index
-                        REQUIRE(create_shapefile_index(path, index_parts) == 0);
+                        if (feature_count > 0)
-                        if (feature_count == 0)
                         {
+                            REQUIRE(create_shapefile_index(path, index_parts) == EXIT_SUCCESS);
+                        }
+                        else
+                        {
+                            REQUIRE(create_shapefile_index(path, index_parts) != EXIT_SUCCESS);
                             REQUIRE(!mapnik::util::exists(index_path)); // index won't be created if there's no features
                         }
                         // count features

utils/shapeindex/shapeindex.cpp

@@ -281,7 +281,8 @@ int main (int argc,char** argv)
         }
         else
         {
-            std::clog << "No non-empty geometries in shapefile" << std::endl;
+            std::clog << "Failed to read any features from \"" << filename << "\"" << std::endl;
+            return EXIT_FAILURE;
         }
     }