--- a/testing/mozharness/mozharness/mozilla/structuredlog.py
+++ b/testing/mozharness/mozharness/mozilla/structuredlog.py
@@ -42,25 +42,20 @@ class StructuredOutputParser(OutputParse
     def _get_mozlog_module(self):
         try:
             import mozlog
         except ImportError:
             self.fatal("A script class using structured logging must inherit "
                        "from the MozbaseMixin to ensure that mozlog is available.")
         return mozlog
 
-    def _handle_unstructured_output(self, line):
-        if self.strict:
-            self.critical(("Test harness output was not a valid structured log message: "
-                          "\n%s") % line)
-            self.update_levels(TBPL_FAILURE, log.CRITICAL)
-            return
+    def _handle_unstructured_output(self, line, log_output=True):
+        self.log_output = log_output
         super(StructuredOutputParser, self).parse_single_line(line)
 
-
     def parse_single_line(self, line):
         """Parses a line of log output from the child process and passes
         it to mozlog to update the overall status of the run.
         Re-emits the logged line in human-readable format.
         """
         level = INFO
         tbpl_level = TBPL_SUCCESS
 
@@ -69,22 +64,32 @@ class StructuredOutputParser(OutputParse
             candidate_data = json.loads(line)
             if (isinstance(candidate_data, dict) and
                 'action' in candidate_data and candidate_data['action'] in self.log_actions):
                 data = candidate_data
         except ValueError:
             pass
 
         if data is None:
-            self._handle_unstructured_output(line)
+            if self.strict:
+                self.critical(("Test harness output was not a valid structured log message: "
+                              "\n%s") % line)
+                self.update_levels(TBPL_FAILURE, log.CRITICAL)
+            else:
+                self._handle_unstructured_output(line)
             return
 
         self.handler(data)
 
         action = data["action"]
+        if action == "process_output":
+            # Run process output through the error lists, but make sure the super parser
+            # doesn't print them to stdout (they should go through the log formatter).
+            self._handle_unstructured_output(data['data'], log_output=False)
+
         if action == "log":
             level = getattr(log, data["level"].upper())
 
         log_data = self.formatter(data)
         if log_data is not None:
             self.log(log_data, level=level)
             self.update_levels(tbpl_level, level)