give user feedback for empty or invalid PDF exports

Collectors used to return an empty page list for a missing page, a missing
namespace or an empty selection, which then failed silently or crashed

give user feedback for empty or invalid PDF exports

Collectors used to return an empty page list for a missing page, a missing
namespace or an empty selection, which then failed silently or crashed when the
first page was accessed. They now throw a translated ExportException instead,
which the action handler turns into feedback: a flash message and redirect for
regular navigations, or an HTTP error with the message in the body for the
background download BookCreator uses. Dynamic message parts are HTML escaped
because that body is injected into the page via a jQuery html() sink.

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rector and codesnifffer fixes

extract $INPUT dependencies out of Collector classes

This should make tests and a future command line interface easier to
implement.

fixed issues from first real run

more refactoring

The basic handling of page collecting, caching and rendering is now
defined.

There is still some ugly dependency in the renderer and the
ImageProcessor loading is not reimplemented

more refactoring

The basic handling of page collecting, caching and rendering is now
defined.

There is still some ugly dependency in the renderer and the
ImageProcessor loading is not reimplemented yet.

After the above a first attempt to actually run the code can be made.

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move page collecting into separate classes