R1:b5bed7b0fa87

Make omitting "value" from a transaction description an explicit error

Summary: See PHI1710. Until D21044, some transactions could omit "value" and apply correctly. This now throws an exception when accessing `$xaction['value']`. All transactions are expected to have a "value" key, so require it explicitly rather than implicitly.

Test Plan: Submitted a transaction with a "type" but no "value". After D21044, got a language-level exception. After this change, got an explicit exception.

Differential…