add a paramter to control page-load-termination-on-JS-error

A previous change terminates page load if there's certain JS errors;
the intention was to speed up layout tests such that page accessing
non-existent test controllers/methods will get immediate termination
instead of waiting on timeout. However this causes problem for page
cycler because it may interrupt the test run too early. Also there
was a bug on bracketing on the termination conditions (operator
precendence issue).

Change-Id: I2f19e48fa0061286fddf0f7cbb4953d7f7d88f76