add check that dash was correctly imported and exit with helpful message if failed

[ned2]

A common gotcha that people new to Dash encounter is naming their script 'dash.py' which then shadows the Dash package itself, causing unhappy times. This adds an explicit check that the dash.development attribute exists, which if it doesn't, it's likely this situation has happened, so a more helpful error message is printed to standard error, and the script exits.

This catches at least some of the times that this error occurs. If we also change the import order for example code from the Dash User guide to make from dash.dependencies import Input, State, Output come after after the import of dash_core_components this will then catch a lot more.

[ned2]

This mirrors the changes in both plotly/dash-html-components#39 and plotly/dash-components-archetype#42

[chriddyp]

perfect 👌