Log the file scope in debug command

[Arcanemagus]

Display the scopes of the last cursor in the current file in the debug command so they can be verified against the scopes allowed in the configuration of the package.

Also guards against an invalid TextEditor somehow getting returned from atom.workspace.getActiveTextEditor().

[Arcanemagus]

Example debug output from this modification:

Atom version: 1.15.0-beta4
linter-eslint version: 8.1.2
ESLint version: 3.16.1
Hours since last Atom restart: 0
Platform: win32
Using local project ESLint from: C:\Users\Landon Abney\Documents\GitHub-release\linter-eslint\node_modules\eslint
Current file's scopes: [
  "source.js",
  "string.quoted.template.js",
  "source.js.embedded.source",
  "meta.method-call.js",
  "meta.arguments.js",
  "variable.other.property.js"
]
linter-eslint configuration: {
  "disableWhenNoEslintConfig": false,
  "lintHtmlFiles": false,
  "useGlobalEslint": false,
  "showRuleIdInMessage": true,
  "eslintrcPath": "",
  "globalNodePath": "",
  "advancedLocalNodeModules": "",
  "eslintRulesDir": "",
  "disableEslintIgnore": false,
  "disableFSCache": false,
  "fixOnSave": false,
  "scopes": [
    "source.js",
    "source.jsx",
    "source.js.jsx",
    "source.babel",
    "source.js-semantic"
  ],
  "rulesToSilenceWhileTyping": [],
  "rulesToDisableWhileFixing": []
}

[IanVS]

LGTM!