Import error library into site_wifitest

site_wifitest.py has historically used error.TestFail
exceptions without actually importing this class.  This
means whenever there's a TestFail, we get an exception
saying "error" doesn't exit, instead of the root cause.

BUG=none
TEST=ReRun run_remote_tests

Review URL: http://codereview.chromium.org/3421011
1 file changed