Add context to translatable watchdog strings

Since watchdog entries are meant for a more tech-savvy audience I suggest to flag these strings with a distinct context so translators can take this into account.

Sorry, but atm I do not have a box with a diff available, so I cannot provide a patch file.

updater.inc, Line 175:
throw new UpdaterException(t('Unable to parse info file.', NULL, array('context' => 'watchdog')));