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