Report Per-Object Slicing Errors in the CLI

G-code generation collects errors raised per object, such as an empty
first layer, into one SlicingErrors exception whose own message is just
"Errors". The CLI's generic handler printed that word and recorded the
generic slicing error text, so a headless caller had nothing to act on.

Let Print render the per-object messages with each object's name, and have
the CLI catch SlicingErrors ahead of the generic handler, print that text
and record it as the result's error string. The exit code is unchanged. A
unit test lifts a cube off the bed and checks the message names the object.

tests/fff_print/test_print.cpp

@@ -505,3 +505,29 @@ TEST_CASE("Sequential printing publishes the nozzle group result", "[Print][Mult
         CHECK(gcode.find("; SEQ-ND-OK") != std::string::npos);
     }
 }
+
+TEST_CASE("Slicing errors are reported per object with the object's name", "[Print]")
+{
+    Print print;
+    Model model;
+    init_print({Slic3r::Test::cube(20.)}, print, model);
+    // Lift the cube off the bed: its first layer is empty, which G-code export reports per object.
+    ModelObject *object = model.objects.front();
+    object->name = "floating cube";
+    object->instances.front()->set_offset(object->instances.front()->get_offset() + Vec3d(0., 0., 2.));
+    print.apply(model, DynamicPrintConfig::full_print_config());
+    print.set_status_silent();
+
+    ScopedTemporaryFile temp(".gcode");
+    std::string message;
+    try {
+        print.process();
+        print.export_gcode(temp.string(), nullptr, nullptr);
+        FAIL("slicing did not report the empty first layer");
+    } catch (const SlicingErrors &errors) {
+        REQUIRE(errors.errors_.size() == 1);
+        message = print.slicing_errors_message(errors);
+    }
+    CHECK(message.rfind("floating cube: ", 0) == 0);
+    CHECK(message.find("empty first layer") != std::string::npos);
+}