add exception handling to deprecated v1 credential support

see: https://github.com/ansible/tower/issues/1268

awx/api/fields.py

@@ -3,12 +3,14 @@
 
 # Django
 from django.utils.translation import ugettext_lazy as _
+from django.core.exceptions import ObjectDoesNotExist
 
 # Django REST Framework
 from rest_framework import serializers
 
 # AWX
 from awx.conf import fields
+from awx.main.models import Credential
 
 __all__ = ['BooleanNullField', 'CharNullField', 'ChoiceNullField', 'VerbatimField']
 
@@ -87,3 +89,20 @@ class OAuth2ProviderField(fields.DictField):
         if invalid_flags:
             self.fail('invalid_key_names', invalid_key_names=', '.join(list(invalid_flags)))
         return data
+
+
+class DeprecatedCredentialField(serializers.IntegerField):
+
+    def __init__(self, **kwargs):
+        kwargs['allow_null'] = True
+        kwargs['default'] = None
+        kwargs['min_value'] = 1
+        kwargs['help_text'] = 'This resource has been deprecated and will be removed in a future release'
+        super(DeprecatedCredentialField, self).__init__(**kwargs)
+
+    def to_internal_value(self, pk):
+        try:
+            Credential.objects.get(pk=pk)
+        except ObjectDoesNotExist:
+            raise serializers.ValidationError(_('Credential {} does not exist').format(pk))
+        return pk