Fix thrown errors breaking out of --watch mode

[JakeDawkins]

Currently, if you write an operation that throws an error in the validation step of codegen while in --watch mode, the watching will stop.

This PR changes a couple things to prevent that.

1. instead of throwing an error in the validation step, we use the error logger from apollo-language-server.
2. Changes the error logger that the CLI sets from using this.error (which exits with a 1 code) to console.error.
3. Updates the watch mode function to catch thrown errors properly, as well as printing how many files are written.

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.