Implement HTTP Cats

Closes #15

src/humulus/app.py

@@ -16,7 +16,7 @@
 
 import os
 
-from flask import Flask
+from flask import Flask, render_template
 
 
 def create_app(test_config=None):
@@ -55,4 +55,22 @@ def create_app(test_config=None):
     from . import filters
     filters.create_filters(app)
 
+    # Register custom error handlers
+    app.register_error_handler(400, bad_request)
+    app.register_error_handler(404, page_not_found)
+
     return app
+
+
+def bad_request(e):
+    return (
+        render_template('_error.html', code=400, message='400 Bad Request'),
+        400
+    )
+
+
+def page_not_found(e):
+    return (
+        render_template('_error.html', code=404, message='404 Not Found'),
+        404
+    )

src/humulus/templates/_error.html

@@ -0,0 +1,24 @@
+{#-
+  Copyright 2019 Mike Shoup
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+-#}
+{% extends '_base.html' %}
+{% block title %}Home{% endblock %}
+
+{% block body %}
+<div class="text-center">
+  <img src="https://http.cat/{{ code }}" alt="{{ message }}">
+  <p>Error messages from <a href="https://http.cat/">HTTP Cats</a></p>
+</div>
+{% endblock %}