conversion: Detect when LibreOffice silently fails

Sometimes, LibreOffice returns with status code 0, but in reality, it
fails. It doesn't create a file, and Dangerzone does not detect this.
What happens next is that it fails in the next command, and throws an
unrelated error.

Detect that LibreOffice fails, by checking if the output file exists,
after the PDF conversion.

dangerzone/conversion/doc_to_pixels.py

@@ -185,6 +185,12 @@ class DocumentToPixels(DangerzoneConverter):
                 timeout=timeout,
             )
             pdf_filename = "/tmp/input_file.pdf"
+            # XXX: Sometimes, LibreOffice can fail with status code 0. So, we need to
+            # always check if the file exists. See:
+            #
+            #     https://github.com/freedomofpress/dangerzone/issues/494
+            if not os.path.exists(pdf_filename):
+                raise ValueError("Conversion to PDF with LibreOffice failed")
         elif conversion["type"] == "convert":
             self.update_progress("Converting to PDF using GraphicsMagick")
             args = [