./bin/run-addon should validate the server argument

[jaredhirsch]

STR:

• clean checkout of screenshots repo (or do make clean)
• do ./bin/run-addon -s screenshots.firefox.com
  • note that the URL does not include the "https://"
• Firefox will start, then immediately crash, with this error:

run: WebExtError: installTemporaryAddon: Error: unknownError: Could not install add-on at '/Users/jhirsch/codez/github/mozilla-services-screenshots/addon/webextension': Error: Extension is invalid
    at /Users/jhirsch/codez/github/mozilla-services-screenshots/node_modules/web-ext/dist/webpack:/src/firefox/remote.js:9:5637
    at Client.handleMessage (/Users/jhirsch/codez/github/mozilla-services-screenshots/node_modules/firefox-client/lib/client.js:161:7)
    at Client.readMessage (/Users/jhirsch/codez/github/mozilla-services-screenshots/node_modules/firefox-client/lib/client.js:220:10)
    at Client.onData (/Users/jhirsch/codez/github/mozilla-services-screenshots/node_modules/firefox-client/lib/client.js:186:16)
    at emitOne (events.js:96:13)
    at Socket.emit (events.js:191:7)
    at readableAddChunk (_stream_readable.js:176:18)
    at Socket.Readable.push (_stream_readable.js:134:10)
    at TCP.onread (net.js:554:20)

[ianb]

This valid could also/instead be validated in bin/build-scripts/update_manifest.py