ApiTestCase: Avoid notices about undefined indices

ApiTestCase: Avoid notices about undefined indices

We're going to throw an exception right away here, but this is still
problematic because of PHPUnit:

"By default, PHPUnit converts PHP errors, warnings, and notices that
are triggered during the execution of a test to an exception."
http://phpunit.de/manual/4.2/en/writing-tests-for-phpunit.html#writing-tests-for-phpunit.errors

Also change the exception message to something saner.

Change-Id: I766c36076d34428035be78bbb3cf36cc572b565b