BUG: Fix crash in transforms on malformed CUSTOM events.

Fixes a crash in various transforms when providing CUSTOM events whose fields
don't match the fields required for the transform.
WrongDataForTransform gets raised if the passed event doesn't have all the required fields for the transform being calculated.  For example. VWAP requires `price` and `volume` fields.  We allow users` `CUSTOM` events to pass through transforms if they supply the appropriate fields, but currently this causes a crash if you provide a custom datasource that doesn't supply the fields of your transform.