Alembic import: be more lenient towards unknown object types.

Alembic is an interchange and caching format, that can contain custom
object schemas. Blender shouldn't crash (because of failing asserts) just
because it doesn't know such an object type.

source/blender/alembic/intern/abc_util.cc

@@ -366,7 +366,10 @@ AbcObjectReader *create_reader(const Alembic::AbcGeom::IObject &object, ImportSe
 		reader = new AbcCurveReader(object, settings);
 	}
 	else {
-		assert(false);
+		std::cerr << "Alembic: unknown how to handle objects of schema "
+		          << md.get("schemaObjTitle")
+		          << ", skipping object "
+		          << object.getFullName() << std::endl;
 	}
 
 	return reader;

source/blender/alembic/intern/alembic_capi.cc

@@ -502,11 +502,10 @@ static bool visit_object(const IObject &object,
 		parent_is_part_of_this_object = true;
 	}
 	else {
-		std::cerr << "object is of unsupported schema type "
-		          << "'" << object.getMetaData().get("schemaObjTitle") << "'"
+		std::cerr << "Alembic object " << full_name
+		          << " is of unsupported schema type '"
+		          << object.getMetaData().get("schemaObjTitle") << "'"
 		          << std::endl;
-		BLI_assert(false);
-		return false;
 	}
 
 	if (reader) {