PB-931 Fix the exception traceback handling in formatter

The exc_text wasn't added to the output message properly, which meant we
couldn't map it with the logging config in django like this:

error:
  stack_trace: exc_text

Author: schtibe

logging_utilities/formatters/json_formatter.py

@@ -343,16 +343,17 @@ def format(self, record):
         if self.add_always_extra:
             extra = self._get_extra_attrs(record)
 
-        message = self.formatMessage(record)
-
-        if self.add_always_extra:
-            self._add_extra_to_message(extra, message)
-
         if record.exc_info:
             # Cache the traceback text to avoid converting it multiple times
             # (it's constant anyway)
             if not record.exc_text:
                 record.exc_text = self.formatException(record.exc_info)
+
+        message = self.formatMessage(record)
+
+        if self.add_always_extra:
+            self._add_extra_to_message(extra, message)
+
         if record.exc_text:
             message['exc_text'] = record.exc_text