mpbb: Remove failcache entry if port is already installed

[ryandesign (Ryan Carsten Schmidt)]

We had an unusual problem where a failcache entry didn't get deleted upon a successful build.

What happened was that ​libsdl built fine but an error occurred during activation so the build was marked as failed. I ​forced another build of libsdl which succeeded and distributed the previously built archive. The install-port part of the build didn't have to do anything; it just printed:

libsdl already installed, nothing to do

So even though the archive was now distributed, the failcache still considered the port unbuildable.

Maybe mpbb should clear the failcache entry when the port was already installed (see patch).

On the other hand, in this case, the failure of libsdl to activate was due to a problem with the tbz2 archive (#61018), and distributing that broken archive isn't helpful.