Does not gracefully handle missing yarn binary

[sandfox]

currently the script assumes that a yarn binary is present at the user supplied path.
If it's missing, or just something else the script fails in various ways

e.g

TypeError: Cannot read property 'trim' of null
    at getLocalYarnVersion (/Users/sandfox/code/ConnectedHomes/project/node_modules/@sandfox/bundled-yarn-updater/index.js:21:71)
    at fetch.then.then.payload (/Users/sandfox/code/ConnectedHomes/project/node_modules/@sandfox/bundled-yarn-updater/index.js:29:28)
    at <anonymous>
    at process._tickCallback (internal/process/next_tick.js:188:7)

We should handle the missing file case with a nice error message, and maybe offer a cli. flag to "force install" (for example when starting a new project).

We should also handle when the binary is either not an executable thing, or something else with a nice error message.