Catch APIException in doc generation (#5443)

The documentation generator calls view.get_serializer() in order to
inspect it for documentation generation. However, if get_serializer()
throws an APIException (e.g. PermissionDenied), it doesn't get caught at
the call site, but instead propagates up and aborts the entire view.
With the try/except in this commit, the documentation generator instead
gratiously ignores that particular view and moves on to the next one
instead. Practical concequences of this commit is that the docs no
longer break if any view's get_serializer(..) throws an APIException.

rest_framework/schemas/inspectors.py

@@ -4,13 +4,14 @@ inspectors.py   # Per-endpoint view introspection
 See schemas.__init__.py for package overview.
 """
 import re
+import warnings
 from collections import OrderedDict
 
 from django.db import models
 from django.utils.encoding import force_text, smart_text
 from django.utils.translation import ugettext_lazy as _
 
-from rest_framework import serializers
+from rest_framework import exceptions, serializers
 from rest_framework.compat import coreapi, coreschema, uritemplate, urlparse
 from rest_framework.settings import api_settings
 from rest_framework.utils import formatting
@@ -285,7 +286,14 @@ class AutoSchema(ViewInspector):
         if not hasattr(view, 'get_serializer'):
             return []
 
-        serializer = view.get_serializer()
+        try:
+            serializer = view.get_serializer()
+        except exceptions.APIException:
+            serializer = None
+            warnings.warn('{}.get_serializer() raised an exception during '
+                          'schema generation. Serializer fields will not be '
+                          'generated for {} {}.'.format(
+                              type(view), method, path))
 
         if isinstance(serializer, serializers.ListSerializer):
             return [