Work around Swift Foundation returning different error code when executable is unavailable.
Work around Swift Foundation returning different error code when executable is unavailable.

This way "launchThrowably" returns the same error code on macOS and Linux when the executable can not be executed.

https://travis-ci.org/kareman/SwiftShell/jobs/511594470#L575

SwiftShell/Tests/SwiftShellTests/Command_Tests.swift:271: error: RunAndPrint_Tests.testThrowsErrorOnInaccessibleExecutable : failed - Caught error <The operation could not be completed>, but not of the expected type and value <Could not execute file at path 'notachance'.>.

kareman/SwiftShell,kareman/SwiftShell,kareman/SwiftShell