Fix some bugs for rendering upgrade package diffs

in particular, when a new package gets installed and there is no "old"
version for it

tailbone/diffs.py

@@ -48,10 +48,10 @@ class Diff(object):
         return sorted(set(self.old_data) | set(self.new_data), key=lambda x: x.lower())
 
     def old_value(self, field):
-        return self.old_data[field]
+        return self.old_data.get(field)
 
     def new_value(self, field):
-        return self.new_data[field]
+        return self.new_data.get(field)
 
     def values_differ(self, field):
         return self.new_value(field) != self.old_value(field)

tailbone/views/upgrades.py

@@ -28,6 +28,7 @@ from __future__ import unicode_literals, absolute_import
 
 import os
 import re
+import logging
 
 import six
 from pip.download import PipSession
@@ -46,6 +47,9 @@ from tailbone.views import MasterView3 as MasterView
 from tailbone.progress import SessionProgress, get_progress_session
 
 
+log = logging.getLogger(__name__)
+
+
 class UpgradeView(MasterView):
     """
     Master view for all user events
@@ -227,6 +231,7 @@ class UpgradeView(MasterView):
                                class_='showing')
             return showing + diff.render_html()
         except:
+            log.exception("failed to render package diff for upgrade: {}".format(upgrade))
             return "(not available for this upgrade)"
 
     def changelog_link(self, project, url):
@@ -264,6 +269,8 @@ class UpgradeView(MasterView):
         return field
 
     def render_diff_value(self, field, value):
+        if value is None:
+            return ""
         if value.startswith("u'") and value.endswith("'"):
             return value[2:1]
         return value