fix(endpoints): added in proper error handling

before we would return a 400 without a message because the errors were not being caught

Issue: https://www.pivotaltracker.com/story/show/145459707

- [ ] It works!
- [ ] Comments provide sufficient explanations for the next contributor
- [ ] Tests cover changes and corner cases
- [ ] Follows Quay syntax patterns and format

test/test_api_usage.py

@@ -4834,6 +4834,20 @@ class TestRepositoryManifestLabels(ApiTestCase):
 
     self.assertEquals(0, len(json['labels']))
 
+    self.postJsonResponse(RepositoryManifestLabels,
+                          params=dict(repository=repository,
+                                      manifestref=tag_manifest.digest),
+                          data=dict(key='bad_label', value='world',
+                                    media_type='text/plain'),
+                          expected_code=400)
+
+    self.postJsonResponse(RepositoryManifestLabels,
+                          params=dict(repository=repository,
+                                      manifestref=tag_manifest.digest),
+                          data=dict(key='hello', value='world',
+                                    media_type='bad_media_type'),
+                          expected_code=400)
+
     # Add some labels to the manifest.
     with assert_action_logged('manifest_label_add'):
       label1 = self.postJsonResponse(RepositoryManifestLabels,