Use chrome.runtime.lastError.message for rendering socket errors.

This is a built-in textual string. For additional information,
also still include the error code (turned into a positive number).
Use this in all calls in both ChromeSocket and RemoteDevice.

Resolves issue #193.
Merge pull request #202 from tvierling/textual_errors

Use chrome.runtime.lastError.message for rendering socket errors.