Report exceptions raised in document conversion

Exceptions raised during the document conversion process would be
silently hidden. This was because ThreadPoolExecuter in logic.py created
various contexts and hid any exceptions raised.

Fixes #309

dangerzone/isolation_provider/base.py

@@ -26,7 +26,13 @@ class IsolationProvider(ABC):
         stdout_callback: Optional[Callable] = None,
     ) -> None:
         document.mark_as_converting()
-        success = self._convert(document, ocr_lang, stdout_callback)
+        try:
+            success = self._convert(document, ocr_lang, stdout_callback)
+        except Exception:
+            success = False
+            log.exception(
+                f"An exception occurred while converting document '{document.id}'"
+            )
         if success:
             document.mark_as_safe()
             if document.archive_after_conversion: