build.excludes and build.includes fail silently

[swashbuck]

Subject of the issue

In config.json, you can exclude or include plugins with the build object:

"build": {
  "excludes": [
    "adapt-blunderbuss"
  ]
},

The issue is that there isn't a warning or error message if the plugin isn't found. This can lead to mistakes when developers assume that a misspelled plugin is being excluded or included.

"build": {
  "excludes": [
    "adapt-contrib-asessment"
  ]
},

Or (no "contrib"):

"build": {
  "excludes": [
    "adapt-assessment"
  ]
},

Expected behaviour

It would be useful to have a warning that appears on command line after you build a course. For example:

build.excludes warning: adapt-contrib-asessment plugin not found