Implement pending.wrap helper method
Make nextTick cancelable using pending.wrap
Implement safeExec helper method
Move loadInContext exec to pending events
Add test for failure for script exec cases
Cleanup var decls a bit
Halt execution of the script loader on error

This also ties the script loader into the pending event architecture so we can be certain that all scripts execute in the event of calling emit early in the page lifecycle.
Properly handle errors in onEmit callbacks