Crash when docker isn't installed on client

[edvald]

Bug

Current Behavior

When running garden deploy for a local environment with the docker CLI installed, garden crashes:

 garden-example-vote-helm (garden-example-vote-helm→pin-redis-version) garden deploy
Deploy 🚀

⠸ providers                 → Preparing environment...
    ⠹ local-kubernetes          → Configuring...
       ⠹ kubernetes-dashboard      → Building version v-ea099d73da...
       ⠋ ingress-controller        → Building version v-f78df59ea8...
       ⠇ default-backend           → Getting build status...

events.js:167
      throw er; // Unhandled 'error' event
      ^

Error: spawn docker ENOENT
    at Process.ChildProcess._handle.onexit (internal/child_process.js:232:19)
    at onErrorNT (internal/child_process.js:407:16)
    at process._tickCallback (internal/process/next_tick.js:63:19)
Emitted 'error' event at:
    at Process.ChildProcess._handle.onexit (internal/child_process.js:238:12)
    at onErrorNT (internal/child_process.js:407:16)
    at process._tickCallback (internal/process/next_tick.js:63:19)

Expected behavior

First off, exceptions of this nature should not crash like this, but rather fail the surrounding task and not crash the process.
Second, it should give a clear error telling the user that docker needs to be installed. Or we could install the docker client automatically, same way we do kubectl, helm etc.

Reproducible example

Make sure docker is not on your PATH. In the garden-example-vote-helm, run garden deploy.

Workaround

Install docker :P

Your environment

Latest garden master. No kubectl or docker installed.