Refactor Error Modal to read properties from GeppettoConfiguration.json

[jrmartin]

The Error Modal needs to be configurable by the geppetto-application using it. To do this, the ErrorModal needs to read the values (error messages, links, button visibility) from the GeppettoConfiguration.json. If not added to the GeppettoConfiguration.json, a default set of properties declared in ErrorModal will be used.

ErrorModal properties added to GeppettoConfiguration.json under existing object "properties".

  "properties": {
    "errorDialog": {
      "message" : "I broke geppetto, ops!",
      "githubButton": {
        "enabled" : true,
        "url" : "https://github.com/openworm/org.geppetto/issues/new"
      },
      "twitterButton": {
        "enabled" : true,
        "url" : "https://geppetto.org",
        "message" : "I broke geppetto, ops!"
      }
    }

PR #111 showing the changes needed in ErrorModal.js