Investigate if the error handling in the action processor can be simplified

[coreymckrill]

Part of the problem in #1245 was due to a try/catch block that intercepts all throwables (which, in modern versions of PHP includes things like fatal errors for calling undefined functions) but then doesn't always handle them in a way that's useful for debugging. In another discussion elsewhere, it was explained like this:

...the overly complex error handling structure we have in place (a custom error handler, followed by a try/catch inside a try/catch/finally) may not be needed any longer. This was always a bit convoluted, but existed partly because we were trying to handle a very wide range of PHP versions, with some massive differences in terms of which errors were throwable and which were not. Since that was introduced, though, we have subsequently dropped support for PHP 5.6 ... so it may be viable to drastically simplify it now.

We should investigate whether this is indeed viable, and simplify it if we can. Our future selves will thank us.