Less aggressive Reloader alert

[chriddyp]

Currently if an app fails, the reloader alert message is pretty aggressive. If you close it, it will re-appear almost immediately.

We should only show the alert once per run.

[chriddyp]

Here is the code: https://github.com/plotly/dash/blob/dev/dash/dash-renderer/src/components/core/Reloader.react.js#L157-L169