In doing so, discovered that if we make `$out` required in v2, there's no need
to introduce the `setFinalHandler()` argument, as it can be passed directly, or
handled via `Zend\Diactoros\Server::listen()`.

First step in the journey to remove the FinalHandler.

- Bump minimum supported PHP version to 5.6 (allows calling callables without
  `call_user_func()`, and using `::class` notation).
- `NotFoundHandler` returns a basic 404 response, always.
- `ErrorHandler`:
  - creates a PHP error handler that will throw any errors not in the
    current error_handler mask as ErrorExceptions.
  - Does a try/catch around `$next()`
  - Raises a new exception, `MissingResponseException`, if `$next` did
    not return a response.
  - Casts all Throwable/Exception items to error responses:
    - If a boolean true flag is passed to the constructor, it will use
      the stack trace in the response.
    - Triggers listeners on the instance with the exception/throwable,
      request, and response, for reporting purposes.

In order to return a response, each middleware type accepts a prototype
`ResponseInterface` instance to the constructor; this is then used to generate
the returned response in 404/error conditions.

We need to notify users that `$out` cannot be omitted starting with
version 2.0.0.

- Space around operators
- Space after !

- Created a protected method, `createErrorResponse()`, for creating and
  returning the error response to use.
- Made `$isDevelopmentMode` protected, to allow extending classes to
  query its value.

Per @vaclavvanik, this patch:

- Moves the error generation aspects to a new class,
  `ErrorResponseGenerator`.
- Updates the `ErrorHandler`:
  - to allow passing an optional, callable response generator via the
    constructor, instead of the `$isDevelopmentMode` flag.
  - to specify `ErrorResponseGenerator` in production mode if no
    generator is provided during instantiation.
  - to trigger the response generator from `handleThrowable()` in order
    to get an error response, passing the exception, request, and
    response prototype.
  - Marks the class final.

This allows flexibility in generating error responses without requiring
extension, by composing the generator in the handler itself.

Per @michaelmoussa

Spotted by @vaclavvanik