runlock: unhandled error case when 'exec' does return

[bAndie91]

https://github.com/apenwarr/gitbuilder/blob/bf70974a8e70b544b771b30b0849e12a398887ff/runlock#L42

hi i noticed it is not handled when exec does return. see perldoc:

It fails and returns false only if the command does not exist

so NOTREACHED is actually reached sometime

cheers.

[apenwarr]

Yeah, that's a good point, though it doesn't come up very often I guess :)