Clean up write_error.

This commit alters write_error to pull the message from a
get_message function of the error, else it using a default Unexpected
Error string.

sprockets/mixins/json_error/__init__.py

@@ -9,6 +9,7 @@ __version__ = '.'.join(str(v) for v in version_info)
 
 
 class JsonErrorMixin(object):
+    """Mixin to write errors as JSON."""
 
     def write_error(self, status_code, **kwargs):
         """Suppress the automatic rendering of HTML code upon an error.
@@ -22,15 +23,15 @@ class JsonErrorMixin(object):
                 object.
 
         """
-        if kwargs.get('error'):
-            raised_error = kwargs.get('error')
-        else:
-            _, raised_error, _ = kwargs['exc_info']
+        _, raised_error, _ = kwargs.get('exc_info', (None, None))
 
-        error_message = getattr(
-            raised_error, 'log_message', 'Unexpected Error')
         error_type = getattr(raised_error, 'error_type', self._reason)
 
+        try:
+            error_message = raised_error.get_message()
+        except AttributeError:
+            error_message = 'Unexpected Error'
+
         self.error = {
             'message': error_message,
             'type': error_type,
@@ -38,8 +39,5 @@ class JsonErrorMixin(object):
         if hasattr(raised_error, 'documentation_url'):
             self.error['documentation_url'] = raised_error.documentation_url
 
-        error_status_code = getattr(raised_error, 'status_code', status_code)
-        self.set_status(error_status_code)
-
         self.set_header('Content-Type', 'application/json; charset=UTF-8')
         self.finish(self.error)

tests.py

@@ -21,10 +21,12 @@ class CustomExceptionRequestHandler(
     class FailureError(Exception):
 
         status_code = 400
-        log_message = 'Too much Foo'
         error_type = 'FailureError'
         documentation_url = 'http://www.example.com'
 
+        def get_message(self):
+            return 'Too much Foo'
+
     def get(self):
         raise self.FailureError()
 
@@ -43,7 +45,7 @@ class TestHTTPError(testing.AsyncHTTPTestCase):
 
     def test_tornado_thrown_exception(self):
         response = self.fetch('/')
-        expected = {'message': 'Error Reason', 'type': 'Bad Request'}
+        expected = {'message': 'Unexpected Error', 'type': 'Bad Request'}
         self.assertEqual(json.loads(response.body), expected)