UpdatePost no longer uses StoreResult.

And several bugs fixed along the way…

* Several Actions refactored to use more straightforward `#execute`
method, with a single path for success and an exception-triggered error
return;
* PostsController methods updated to match changes in Actions, usually
resulting in simpler code (with the notable exception of
`#on_create_failure`);
* Specs updated to match the above.

RSpec: 755 examples, 0 failures
RuboCop: 178 files inspected, no offenses detected