Reject the processing if the errorThrown property has been set.

This allows aborting the processing queue via data.abort() call.
Use ProgressEvent instead of XMLHttpRequestUpload to check for XHR2 capabilities. Closes #2671
Catch imagick image detection exceptions.
Provide a data.processing() method to determine the processing state of a file selection.
Render the upload template before processing the files.

This allows to abort the processing queue via the user interface.
Merge branch 'master' into gh-pages