log skipped registration
do not cache the base product upgrade, always do the upgrade (bnc#882039)

If the user goes back in the upgrade workflow and selects
a different partition to upgrade then we need to upgrade
a different subscription.

And according to the SCC team a repeated upgrade call is OK
when the same partition is used.
fixed error code handling, hint for failed SLE11 upgrade (bnc#885506)
use "E-mail Address" label (bnc#888158)
handle missing SSL certificate attributes (bnc#886850)
added SSL error translation
log the old credentials for easier debugging
release (3.1.91)
code review fixes
updated Registration::Helpers.registration_url test
Merge pull request #133 from yast/upgrade_fixes

Upgrade fixes