Fix Python error when ManyToMany relations didn't exist.

This avoids storing an ugettext value in self.version.
It's value got parsed in `ids = [int(v) for v in self.value]`
causing an `int("F")` call that fails.

Instead, a sentinal value is introduced that can be recognized in the
code, yet output a meaningful message when it's shown to the user.

Fixes jedie#44

reversion_compare/compare.py

@@ -14,6 +14,7 @@
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import ObjectDoesNotExist
 from django.db import models
+from django.utils.encoding import force_text, python_2_unicode_compatible
 from django.utils.translation import ugettext as _
 
 
@@ -23,6 +24,18 @@
 logger = logging.getLogger(__name__)
 
 
+@python_2_unicode_compatible
+class FieldVersionDoesNotExist(object):
+    """
+    Sentinel object to handle missing fields
+    """
+
+    def __str__(self):
+        return force_text(_("Field Didn't exist!"))
+
+DOES_NOT_EXIST = FieldVersionDoesNotExist()
+
+
 class CompareObject(object):
     def __init__(self, field, field_name, obj, version, has_int_pk, adapter):
         self.field = field
@@ -32,7 +45,7 @@ def __init__(self, field, field_name, obj, version, has_int_pk, adapter):
         self.has_int_pk = has_int_pk
         self.adapter = adapter
         # try and get a value, if none punt
-        self.value = version.field_dict.get(field_name, _("Field Didn't exist!"))
+        self.value = version.field_dict.get(field_name, DOES_NOT_EXIST)
 
     def _obj_repr(self, obj):
         # FIXME: How to create a better representation of the current value?
@@ -122,6 +135,9 @@ def get_many_to_many(self):
         """
         if self.field.get_internal_type() != "ManyToManyField":  # FIXME!
             return ([], [], [], [])  # TODO: refactory that
+        elif self.value is DOES_NOT_EXIST:
+            return ([], [], [], [])
+
         ids = None
         if self.has_int_pk:
             ids = [int(v) for v in self.value]  # is: version.field_dict[field.name]