Cannot convert object to primitive value when ember-cli-deploy-slack not installed but specified in deploy.js

Hello,

I had been ripping small amounts of hair out over the past few days trying to get lightning deploys working again for our teams. I finally realized that I had forgotten to install the Slack plugin. The fix was simple (`ember install ember-cli-deploy-slack`) but the pain was real. Here's what this error looks like:

```
17:23 $ ember deploy:list stage
version: 1.13.15
Cannot convert object to primitive value
TypeError: Cannot convert object to primitive value
at String (native)
at Function.applyStyle (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli/node_modules/chalk/index.js:57:45)
at Chalk.builder (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli/node_modules/chalk/index.js:40:21)
at writeError (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli/lib/ui/write-error.js:19:24)
at UI.writeError (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli/lib/ui/index.js:92:3)
at Class.module.exports.Task.extend._configuredPlugins (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli-deploy/lib/tasks/pipeline.js:115:15)
at Class.module.exports.Task.extend.setup (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli-deploy/lib/tasks/pipeline.js:37:22)
at Class.module.exports.Task.extend.run (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli-deploy/lib/tasks/pipeline.js:87:10)
at /Users/spetrow/projects/music_rights_ui/node_modules/ember-cli-deploy/lib/commands/list.js:51:23
at lib$rsvp$$internal$$tryCatch (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli/node_modules/rsvp/dist/rsvp.js:1036:16)
at lib$rsvp$$internal$$invokeCallback (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli/node_modules/rsvp/dist/rsvp.js:1048:17)
at /Users/spetrow/projects/music_rights_ui/node_modules/ember-cli/node_modules/rsvp/dist/rsvp.js:331:11
at lib$rsvp$asap$$flush (/Users/spetrow/projects/music_rights_ui/node_modules/ember-cli/node_modules/rsvp/dist/rsvp.js:1198:9)
at process._tickCallback (node.js:355:11)

```

The fix:

```
17:23 $ ember install ember-cli-deploy-slack
version: 1.13.15
Installed packages for tooling via npm.
Installed addon package.
✔ ~/projects/music_rights_ui [staging|✚ 2…1]
17:24 $ ember deploy:list stage
version: 1.13.15
- Listing revisions for key: `music-rights-ui-stage:index`
- revision
- ===========
- > 4837452
- c76ef39
```

Package.json:

Versions:

```
✔ ~/projects/music_rights_ui [staging|✚ 2…1]
17:24 $ node --version
v0.12.7
✔ ~/projects/music_rights_ui [staging|✚ 2…1]
17:30 $ ember --version
version: 1.13.15
node: 0.12.7
npm: 2.14.10
os: darwin x64
```

This also happened on our Jenkins CI which runs:

```
npm install
bower install

ember deploy ${EMBER_ENVIRON} --verbose --activate
```

Even on latest stable node (`v5.7.1 (npm v3.6.0)` ) it happens when using this deploy.js:

```
var VALID_DEPLOY_TARGETS = [ //update these to match what you call your deployment targets
'dev',
'test',
'stage',
'prod'
];

module.exports = function(deployTarget) {
var PROJECT_NAME = 'ember-cli-skeleton'; // TODO change me when settting up a new BOLT project

var ENV = {
build: {},
redis: {
// docs https://github.com/ember-cli-deploy/ember-cli-deploy-redis
host: '[REDACTED]',
allowOverwrite: true,
keyPrefix: PROJECT_NAME + ':index'
},
s3: {
accessKeyId: process.env.AWS_KEY,
secretAccessKey: process.env.AWS_SECRET,
bucket: '[REDACTED]',
region: 'us-west-2',
prefix: PROJECT_NAME
},
slack: {
webhookURL: "[REDACTED]'',
username: 'ember-cli-deploy via BOLT',
iconEmoji: ':zap:'
}
};

ENV["revision-data"] = {
//filePattern: '**/*.{html,js,css,png,gif,ico,jpg,map,xml,txt,svg,swf,eot,ttf,woff,woff2}',
type: 'git-commit'
};

if (VALID_DEPLOY_TARGETS.indexOf(deployTarget) === -1) {
throw new Error('Invalid deployTarget ' + deployTarget);
}

if (deployTarget === 'dev') {
ENV.build.environment = 'development';
ENV.redis.keyPrefix = PROJECT_NAME + '-dev:index';
ENV.s3.prefix = PROJECT_NAME + '-dev';
ENV.plugins = ['build', 'display-revisions', 'gzip', 's3', 'redis', 'revision-data', 'slack']; // everything except manifest since manifest breaks when asset versioning is d$
}

if (deployTarget === 'test') {
ENV.build.environment = 'test';
ENV.redis.keyPrefix = PROJECT_NAME + '-test:index';
ENV.s3.prefix = PROJECT_NAME + '-test';
ENV.plugins = ['build', 'display-revisions', 'gzip', 's3', 'redis', 'revision-data', 'slack']; // everything except manifest since manifest breaks when asset versioning is d$
}

if (deployTarget === 'stage') {
ENV.build.environment = 'staging';
ENV.redis.keyPrefix = PROJECT_NAME + '-stage:index';
ENV.s3.prefix = PROJECT_NAME + '-stage';
ENV.plugins = ['build', 'display-revisions', 'gzip', 's3', 'redis', 'revision-data', 'slack']; // everything except manifest since manifest breaks when asset versioning is d$
}

if (deployTarget === 'prod') {
ENV.build.environment = 'production';
ENV.redis.keyPrefix = PROJECT_NAME; // this is the same as we set in the config above (ENV.redis and ENV.s3), just here for clarity
ENV.s3.prefix = PROJECT_NAME;
}

return ENV;
};
```

...and this package.json

```
{
"name": "music-rights-ui",
"version": "0.0.0",
"description": "Small description for music-rights-ui goes here",
"private": true,
"directories": {
"doc": "doc",
"test": "tests"
},
"scripts": {
"build": "ember build",
"start": "ember server",
"test": "ember test"
},
"repository": "",
"engines": {
"node": ">= 0.10.0"
},
"author": "",
"license": "MIT",
"devDependencies": {
"basscss": "^6.1.6",
"basscss-sass": "^1.0.2",
"broccoli-asset-rev": "^2.2.0",
"clone": "1.0.2",
"ember-cli": "^1.13.15",
"ember-cli-accounting": "1.0.0",
"ember-cli-app-version": "^1.0.0",
"ember-cli-babel": "^5.1.5",
"ember-cli-bootstrap-sassy": "0.5.1",
"ember-cli-coffeescript": "0.11.0",
"ember-cli-content-security-policy": "0.4.0",
"ember-cli-datepicker": "2.0.1",
"ember-cli-dependency-checker": "^1.1.0",
"ember-cli-deploy": "0.6.0",
"ember-cli-deploy-lightning-pack": "0.5.0",
"ember-cli-emblem": "0.3.1",
"ember-cli-htmlbars": "^1.0.1",
"ember-cli-htmlbars-inline-precompile": "^0.3.1",
"ember-cli-ic-ajax": "0.2.1",
"ember-cli-inject-live-reload": "^1.3.1",
"ember-cli-moment-shim": "0.6.2",
"ember-cli-pure": "1.0.1",
"ember-cli-qunit": "^1.0.4",
"ember-cli-release": "0.2.8",
"ember-cli-sass": "^4.0.1",
"ember-cli-selectize": "0.4.2",
"ember-cli-sri": "^1.1.0",
"ember-cli-uglify": "^1.2.0",
"ember-data": "1.13.15",
"ember-disable-proxy-controllers": "^1.0.1",
"ember-export-application-global": "^1.0.4",
"ember-faker": "1.1.0",
"ember-i18n": "4.2.0",
"ember-moment": "4.1.0",
"ember-pikaday": "0.10.0",
"ember-power-select": "0.8.4",
"ember-simple-auth": "1.0.1",
"handlebars-intl": "^1.1.1",
"torii": "^0.6.1"
},
"dependencies": {
"ember-radio-buttons": "^4.0.1"
}
}
```

( **note** that `ember-cli-deploy-slack` is missing)

If nothing else, how can we make this error more useful (e.g. `Missing Ember CLI Deploy plugin: slack`) ? My apologies in advance if this should be opened against ember-cli-deploy directly instead.

Thanks!