[1.5.x] Fixed #20278 -- ensured .get() exceptions do not recurse infinitely

A regression caused by d5b93d3281 made .get() error
reporting recurse infinitely on certain rare conditions. Fixed this by
not trying to print the given lookup kwargs.

Backpatch of 266c0bb23e

tests/modeltests/basic/models.py

@@ -18,3 +18,11 @@ class Article(models.Model):
 
     def __str__(self):
         return self.headline
+
+@python_2_unicode_compatible
+class SelfRef(models.Model):
+    selfref = models.ForeignKey('self', null=True, blank=True,
+                                related_name='+')
+
+    def __str__(self):
+        return SelfRef.objects.get(selfref=self).pk