Makes the DispatchIO initializer that accepts a path failable, reflecting the fact that a relative or non-existent path is invalid.
Makes the DispatchIO initializer that accepts a path failable, reflecting the fact that a relative or non-existent path is invalid.

(Radar 31115994)

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch