[swift-stdlib-tool] Add error for missing directories
[swift-stdlib-tool] Add error for missing directories

In the case the user passes a directory that doesn't exist, this now
errors immediately instead of silently ignoring it.

apple/swift,roambotics/swift,glessard/swift,ahoppen/swift,apple/swift,ahoppen/swift,atrick/swift,glessard/swift,benlangmuir/swift,JGiola/swift,benlangmuir/swift,glessard/swift,JGiola/swift,roambotics/swift,JGiola/swift,apple/swift,roambotics/swift,benlangmuir/swift,atrick/swift,atrick/swift,apple/swift,roambotics/swift,JGiola/swift,apple/swift,apple/swift,benlangmuir/swift,ahoppen/swift,benlangmuir/swift,glessard/swift,atrick/swift,atrick/swift,JGiola/swift,ahoppen/swift,atrick/swift,benlangmuir/swift,JGiola/swift,roambotics/swift,glessard/swift,glessard/swift,ahoppen/swift,ahoppen/swift,roambotics/swift