Manage errors due to invalid PDF files

[laritakr]

When a PDF doesn't have the appropriate information, the PagesIntoImagesService cannot break it into TIFFs.

The service defines invalid_pdf? which specifies all of the information which is needed by the service. This method could be put into use somewhere in ChildWorksFromPdfJob to prevent the job from indefinitely retrying due to the error, and possibly reporting the error as well.