No error message when we launch the app without setting up environment variables

[ntraut]

Currently when we build the app without setting up the required environmental variables, we still are able to launch the app but we can't login. Error messages are shown in the dev console but not in the user interface, which could make it difficult to understand when we have no access to the console. Maybe it would be good to have a visual error message for this case also.

[r03ert0]

For the end user, there should only be a generic error message indicating that we're having a problem ("oh snap!"), and suggesting to try again later.
For us, it would be good to have an e-mail indicating that building failed, like with CircleCI. Once we'll have tests, we should also be prevented of errors by e-mail. Would that be easy to set up?

[ntraut]

Currently the build pass even without env variables set, maybe it would be better if the build failed.
For being notified when the build fails, I think we can set up something like that in app center. We could also use a classic CI on the SPA app.

[ntraut]

Maybe not totally solved but at least i added an error message when the variables for the parse api are not correctly set in commit bcf1d3e.