Validate dependancies are present before deploying

If a user clones down a service and forgets to npm install they can deploy broken services.

They see this ultra vague error in lambda logs

START RequestId: dd920e5d-7ee0-11e7-a6f0-7fe125518660 Version: $LATEST
Unable to import module 'handler': Error
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
END RequestId: dd920e5d-7ee0-11e7-a6f0-7fe125518660

This should be solved by the framework.

1. User runs sls deploy
2. Framework check package.json
3. Framework validates that the node_module folder is present

If node_modules not found, error (or automatically run npm i)

If node_modules good, continue with deploy as usually.

This is an easily solvable issue that causes unnecessary user pain.

[kbariotis]

In the same sense, Serverless could also validate that the handlers defined are present and indeed exporting the required functions.

E.g., given a configuration like this:

functions:
  install:
    handler: src/handlers/install.installHandler
    events:
      - http: GET /

Serverless should verify that src/handlers/install file exists and that also provides the installHandler function.

It will save the developers so much time.

[pmuens]

Closing since this is a duplicate of https://github.com/serverless/serverless/issues/3270.