When no dockerfile present in kube deploy, let them know about init

I sometimes create a project in VS Code and then try to deploy to KEDA with `func kubernetes deploy`. If I've forgotten to generate a dockerfile I get an error:

>Error running docker build -t jeffhollan/queue-function /Users/jeffhollan/Development/Temp/simple-keda-queue.
output:

> unable to prepare context: unable to evaluate symlinks in Dockerfile path: lstat /Users/jeffhollan/Development/Temp/simple-keda-queue/Dockerfile: no such file or directory

Would be nice if it said something like:

> You can generate a Dockerfile for an existing project with the command `func init --docker-only`.

Or maybe even if no Dockerfile it prompts if you want to generate one first