XMLBear: Process error correctly

Errors were shown as logs instead of errors. Corrected the processing of
output to show the error messages.

Fixes #251

bears/xml/XMLBear.py

@@ -22,12 +22,13 @@ class XMLBear(LocalBear, Lint):
     diff_message = "XML can be formatted better."
     output_regex = r'(.*\.xml):(?P<line>\d+): (?P<message>.*)\n.*\n.*'
     gives_corrected = True
+    use_stderr = True
 
     def process_output(self, output, filename, file):
+        if self.stderr_output:
+            return self._process_issues(self.stderr_output, filename)
         if self.stdout_output:  # only yield Result if stdout is not empty
             return self._process_corrected(self.stdout_output, filename, file)
-        if self.stderr_output:  # pragma: no cover
-            return self._process_issues(self.stderr_output, filename)
 
     def run(self, filename, file,
             xml_schema: path="",