Handle exception when RequestHandler::startup() fails.

In the case that there is a request data type parser raises an
exception, or startup() otherwise fails the error page should be created
correctly. While I'm not able to write a test case for this, manual
testing confirmed the fix.

Refs #5311
Add comments for possibly confusing code.
Remove trailing whitespace.
Merge pull request #5406 from cakephp/issue-5311

Handle exception when RequestHandler::startup() fails.

Refs #5311