Generated file path requires all directories to exist

[brentleyjones]

When invoking the following command:

Carthage/Checkouts/Needle/Generator/bin/needle generate App/Sources/Generated/Needle.swift

needle requires that the Sources and Generated directories exist. This can be an issue when the directory is specifically for generated files and is excluded from source control. Currently we have to run mkdir -p App/Sources/Generated before running needle.

Also, the error that is reported was cryptic, and required running needle attached to a debugger to understand what the actual issue was.