Improve error message in the build generator when swift is not on the path

[tunniclm]

This is what the error message looks like now:

? What's the name of your application? testapp
? Enter the name of the directory to contain the project: testapp
identical config.json
identical Package.swift
identical Sources/testapp/main.swift
identical .swiftservergenerator-project
identical definitions/testapp.yaml
events.js:141
      throw er; // Unhandled 'error' event
      ^

Error: spawn swift ENOENT
    at exports._errnoException (util.js:870:11)
    at Process.ChildProcess._handle.onexit (internal/child_process.js:178:32)
    at onErrorNT (internal/child_process.js:344:16)
    at nextTickCallbackWith2Args (node.js:441:9)
    at process._tickCallback (node.js:355:17)