Add error handling for missing regulations

- Addresses issue eregs#485: Missing data leads to an explosion
- Also addresses FEC issue: fecgov/fec-eregs#385

- Throw a 404 error when navigating to a regulation that doesn't exist
- Previously, the `utils.regulation_meta` function was returning `{}`, so accessing the 'meta' data was resulting in a 500 error

regulations/tests/views_chrome_tests.py

@@ -52,6 +52,20 @@ class FakeView(chrome.ChromeView):
     assert response.status_code == 404
 
 
+def test_chrome_empty_meta(monkeypatch, rf):
+    """Return 404 when trying to access missing regulation.
+    In this situation, `regulation_meta` returns {} """
+
+    chrome_view = chrome.ChromeView()
+
+    monkeypatch.setattr(chrome, 'fetch_grouped_history', Mock())
+    monkeypatch.setattr(chrome, 'fetch_toc', Mock(return_value=[]))
+    monkeypatch.setattr(chrome.utils, 'regulation_meta', Mock(return_value={}))
+
+    with pytest.raises(error_handling.MissingContentException):
+        chrome_view.set_chrome_context({}, '2', 'version')
+
+
 def test_chrome_error_propagation(monkeypatch, rf):
     """While we don't rely on this sort of propagation for the main content
     (much), test it in the sidebar"""

regulations/views/chrome.py

@@ -110,6 +110,11 @@ def set_chrome_context(self, context, reg_part, version):
         context['TOC'] = toc
 
         context['meta'] = utils.regulation_meta(reg_part, version)
+
+        # Throw 404 if regulation doesn't exist
+        if not context['meta']:
+            raise error_handling.MissingContentException()
+
         context['version_span'] = version_span(
             context['history'], context['meta']['effective_date'])
         context['version_switch_view'] = self.version_switch_view