Improve strings for messages when fetching updates

As pointed out over at http://drupal.org/node/155878#comment-285842 there are two nearly identical strings for the same thing, one for drupal_set_message(), the other for watchdog(). This makes it slightly inconsistent and a little harder to translate.

Attached patch just fixes the strings to:

A) Use the identical text in both places.

B) Consistently use "information about all available..." instead of "information on all available..." in the related messages when the site failed to fetch info.

Since this is so trivial, I'm just posting this directly as RTBC.