Make the default behavior of getApplicationTransactionCommentObject() "return null" instead of "throw"

Summary:
Depends on D20115. See https://discourse.phabricator-community.org/t/transaction-search-endpoint-does-not-work-on-differential-diffs/2369/.

Currently, getApplicationTransactionCommentObject() throws by default. Subclasses must override it to return null to indicate that they don't support comments.

This is silly, and leads to a bunch of code that does a try / catch around it, and at least some code (here, transaction.search) which doesn't try / catch and gets the wrong behavior as a result.

Just make it return null by default, meaning "no support for comments". Then remove the try / catch stuff and all the return null implementations.

Test Plan:

• Grepped for getApplicationTransactionCommentObject(), fixed each callsite / definition.
• Called transaction.search on a diff with transactions (i.e., not a sourced-from-commit diff).

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: jbrownEP

Differential Revision: https://secure.phabricator.com/D20121