app: Add global error handling

When a controller raises an error, the application now handles it and
ensures that the proper representation is returned to the user-agent.

src/dcow/app.py

@@ -5,8 +5,13 @@ from . import (
     thumbnails,
 )
 import functools
+import logging
 import milla.util
 import os
+import sys
+
+
+log = logging.getLogger(__name__)
 
 
 DEFAULT_CONFIG = {
@@ -53,3 +58,21 @@ class Application(milla.Application):
             request.want = 'xhtml'
         request.ResponseClass = functools.partial(base.VariedResponse, request)
         return request
+
+    def handle_error(self, request):
+        response = request.ResponseClass()
+        e = sys.exc_info()[1]
+        err = {}
+        if isinstance(e, milla.HTTPRedirection):
+            return e
+        elif isinstance(e, milla.WSGIHTTPException):
+            response.status_int = e.code
+            err['code'] = getattr(e, 'errno', e.code)
+            err['message'] = '{}'.format(e)
+        else:
+            log.exception('An unhandled exception occurred:')
+            response.status_int = 500
+            err['code'] = -1
+            err['message'] = 'An unknown error has occurred'
+        response.set_payload('error.html.j2', {'error': err})
+        return response

src/dcow/templates/error.html.j2

@@ -0,0 +1,5 @@
+{% extends "base.html.j2" %}
+{% block body -%}
+<h2>Error</h2>
+<div>{{ error.message }}</div>
+{% endblock body -%}