Simplify error handling for actions, run from handlers

This alters the error-handling so that `runRetconAction` handles
errors and lifts the result of the action into `RetconHandler` as an
`Either RetconError`.

As a result, we can remove much of the error handling code around the
invocation of actions from the core handlers.
Remove stylish-haskell from Makefile

stylish-haskell seems to remove the {-# SOURCE #-} pragmas needed to
use hs-boot files to break import cycles. This makes the code stop
compiling.
Tweak running of actions in canned source tests