Gracefully handle failures to retrieve a flavor

In some cases, a flavor in use can become unaccessible, for instance
after deleting a flavor with access restrictions.

 * On the Instance details page, make the try/except blocks for API
   calls more fine-grained. Some specific failures shouldn't cause the
   whole page to fail to load (and e.g. prevent Console access).

 * Fix the Resize Instance form to display even if information about
   the current flavor cannot be retrieved.

 * Gracefully handle failures to retrieve flavor information on AJAX
   calls for row update, so that the row doesn't disappear from the
   table.

Change-Id: I011b6e01eca3e04e5ef2528732b9bde3782b4eba
Closes-Bug: #1366166
Merge "Gracefully handle failures to retrieve a flavor"