Invalid environment error

[hayes0724]

Add friendly error message when invalid environment is selected, current error:

 packer deploy --env=staging
(node:3528) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'store' of undefined
    at DEFAULT_ENV_VARS.forEach (H:\shopify\tools\shopify-packer\src\env\index.js:27:33)
    at Array.forEach (<anonymous>)
    at assign (H:\shopify\tools\shopify-packer\src\env\index.js:26:20)
    at module.exports (H:\shopify\tools\shopify-packer\cli\commands\deploy.js:13:3)
    at Object.<anonymous> (H:\shopify\tools\shopify-packer\cli\index.js:21:33)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
(node:3528) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejec
tion id: 1)
(node:3528) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.