R1:857e3aee83de

Improve ApplicationTransaction behavior for poorly constructed transactions

Summary:
Ref T2222. Five very small improvements:

- I hit this exception and it took a bit to understand which transaction was causing problems. Add an `Exception` subclass which does a better job of making the message debuggable.
- The `oldValue` of a transaction may be `null`, legitimately (for example, changing the `repositoryPHID` for a revision from `null` to some valid PHID). Do a check to see if `setOldValue()` has…