use lookaheads/lookbehinds to avoid capturing decorative junk

Some install failure patterns include the decorative characters making
up Anaconda's ASCII art (either + characters or Unicode box drawing
characters), as a way of reducing the chance of false positives. However
these look ugly when they appear as the failure message in recipe
results.

We can use zero-width lookahead and lookbehind assertions in the
patterns to match the same strings as before without capturing them.

Change-Id: Idb15a991a3957f59b39f4aa3a1781449287d30c2