Return 400 Bad Request on malformed input JSON

Apparently we didn't have tests for malformed input payload and used to
fall with 500 Internal Server Error if input payload is not
de-serializable. o_O Luckily we found it early and from now on we will
return 400 Bad Request in this case.

tests/test_resource.py

@@ -151,6 +151,16 @@ async def test_post_json(testapp, headers):
     assert await resp.json() == {'who': 'batman'}
 
 
+async def test_post_malformed_payload(testapp):
+    resp = await testapp.post('/test', data='malformed')
+
+    assert resp.status == 400
+    assert await resp.json() == {
+        'message': ('Malformed application/json payload: '
+                    'Expecting value: line 1 column 1 (char 0)'),
+    }
+
+
 async def test_post_unsupported_media_type(testapp):
     resp = await testapp.post(
         '/test',

xsnippet/api/resource.py

@@ -147,7 +147,13 @@ async def impl(self):
 
             if content_type in decoders:
                 decode = decoders[content_type]
-                return decode(await self.text())
+                text = await self.text()
+
+                try:
+                    return decode(text)
+                except Exception as exc:
+                    raise web.HTTPBadRequest(
+                        reason='Malformed %s payload: %s' % (content_type, exc))
 
             raise web.HTTPUnsupportedMediaType()
         return impl