Add error_log to Rackspace exceptions.

This commit logs every error from Rackspace when resolving the URLs.
Additionally it returns a backup url to show the placeholder when something
fails, returning always a valid URL.
Add SERVER_NAME
Test new cases
Include it in coverage.
Revert SERVER_NAME and patch it locally.
Increase coverage.
Increase coverage.
Merge pull request #917 from PyBossa/issue-rackspace-error

Add error_log to Rackspace exceptions.