No longer abort already destroyed FileLoader in FileRepository.

[Mati365]

Suggested merge commit message (convention)

Fix (upload): Editor should no longer crash when executing undo while an image is still being uploaded.

---

Additional information

Closes https://github.com/cksource/ckeditor5-commercial/issues/6817

It looks like undo moves uploaded node to graveyard, which causes abort image uploading here:
https://github.com/ckeditor/ckeditor5/blob/ck/6817/packages/ckeditor5-image/src/imageupload/imageuploadediting.ts#L227

						if ( isInsertedInGraveyard ) {
							// If the image was inserted to the graveyard for good (**not** replaced by another image),
							// only then abort the loading process.
							if ( !insertedImagesIds.has( uploadId ) ) {
								loader.abort();
							}
						} else {

which causes aborted exception, that is caught here:
https://github.com/ckeditor/ckeditor5/blob/ck/6817/packages/ckeditor5-image/src/imageupload/imageuploadediting.ts#L387-L389

			.catch( error => {
				if ( editor.ui ) {
					editor.ui.ariaLiveAnnouncer.announce( t( 'Error during image upload' ) );
				}

				// If status is not 'error' nor 'aborted' - throw error because it means that something else went wrong,
				// it might be generic error and it would be real pain to find what is going on.
				if ( loader.status !== 'error' && loader.status !== 'aborted' ) {
					throw error;
				}

				// Might be 'aborted'.
				if ( loader.status == 'error' && error ) {
					notification.showWarning( error, {
						title: t( 'Upload failed' ),
						namespace: 'upload'
					} );
				}

				// Permanently remove image from insertion batch.
				model.enqueueChange( { isUndoable: false }, writer => {
					writer.remove( imageUploadElements.get( loader.id )! );
				} );

				clean();
			} );

... and tries to abort again, which causes exception. I adjusted FileLoader method to handle similar scenarios, and asseration in image upload.