Improve build errors and show notifications

[geowarin]

This is heavily inspired by the work of @gaearon on create-react-app.

There is an option to disable the OS notifications in the tarec.yml configuration file:

build:
  # show notifications on build failure. Default, true
  showNotification: true

Result:

[gaearon]

Anything here that you would like to contribute back? 😄

[geowarin]

@gaearon Totally! I was thinking about extracting a module for this plugin.
It still needs a bit of work and some tests.
I'll make a PR when it's ready 😄

[coveralls]

Coverage decreased (-1.4%) to 93.261% when pulling d069323 on notifications into 66287ec on master.