throw error if public key is not set, yet

When using `crx` and trying to generate an App ID prior to generating the public key, an error occurs:
```console
  1) lib/autoupdate should build an autoupdate XML file with codebase and with update_url:
     TypeError: Not a string or buffer
      at Hash.update (crypto.js:240:17)
      at Object.ChromeExtension.generateAppId (/Users/joscha/Development/grunt-crx/node_modules/crx/src/crx.js:291:8)
      at Object.ChromeExtension.generateUpdateXML (/Users/joscha/Development/grunt-crx/node_modules/crx/src/crx.js:314:46)
      at generateXML (/Users/joscha/Development/grunt-crx/lib/autoupdate.js:35:25)
      at /Users/joscha/Development/grunt-crx/node_modules/grunt/node_modules/async/lib/async.js:486:21
      at /Users/joscha/Development/grunt-crx/node_modules/grunt/node_modules/async/lib/async.js:185:13
      at iterate (/Users/joscha/Development/grunt-crx/node_modules/grunt/node_modules/async/lib/async.js:108:13)
      at /Users/joscha/Development/grunt-crx/node_modules/grunt/node_modules/async/lib/async.js:119:25
      at /Users/joscha/Development/grunt-crx/node_modules/grunt/node_modules/async/lib/async.js:187:17
      at /Users/joscha/Development/grunt-crx/node_modules/grunt/node_modules/async/lib/async.js:491:34
      at generatePublicKey (/Users/joscha/Development/grunt-crx/lib/autoupdate.js:28:18)
...
```