Build broken: 'adding 'bool' to a string does not append to the string' in filesystem tests

../../src/tests/containerizer/filesystem_isolator_tests.cpp:125:52: error: adding 'bool' to a string does not append to the string [-Werror,-Wstring-plus-int]
        return Error("Failed to create root dir: " + mkdir.isError());
                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
../../src/tests/containerizer/filesystem_isolator_tests.cpp:125:52: note: use array indexing to silence this warning
        return Error("Failed to create root dir: " + mkdir.isError());
                                                   ^
                     &                             [                ]

 124       if (mkdir.isError()) {
 125         return Error("Failed to create root dir: " + mkdir.isError());
 126       }

 124       if (mkdir.isError()) {
 125         return Error("Failed to create root dir: " + mkdir.error());
 126       }