Make :: fail for an invalid dir much like : does for a dir with no BUILD file.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/base/:cmd_line_spec_parser --test-pytest-options=-v
...
17:20:09 00:00   [test]
17:20:09 00:00     [pytest]
17:20:09 00:00       [run]
                     ============== test session starts ===============
                     platform linux2 -- Python 2.6.9 -- py-1.4.23 -- pytest-2.6.1 -- /usr/bin/python2.6
                     plugins: cov, timeout
                     collected 7 items

                     tests/python/pants_test/base/test_cmd_line_spec_parser.py::CmdLineSpecParserTest::test_absolute PASSED
                     tests/python/pants_test/base/test_cmd_line_spec_parser.py::CmdLineSpecParserTest::test_cmd_line_affordances PASSED
                     tests/python/pants_test/base/test_cmd_line_spec_parser.py::CmdLineSpecParserTest::test_cmd_line_spec_list PASSED
                     tests/python/pants_test/base/test_cmd_line_spec_parser.py::CmdLineSpecParserTest::test_does_not_exist PASSED
                     tests/python/pants_test/base/test_cmd_line_spec_parser.py::CmdLineSpecParserTest::test_normal PASSED
                     tests/python/pants_test/base/test_cmd_line_spec_parser.py::CmdLineSpecParserTest::test_sibling PASSED
                     tests/python/pants_test/base/test_cmd_line_spec_parser.py::CmdLineSpecParserTest::test_sibling_or_descendents PASSED

                     ============ 7 passed in 0.10 seconds ============

17:20:43 00:34     [junit]
17:20:43 00:34     [specs]
               SUCCESS

And travis CI went green here: https://travis-ci.org/pantsbuild/pants/builds/32910349

Bugs closed: 484, 492

Reviewed at https://rbcommons.com/s/twitter/r/907/