Handle when no iscomplete correctly in callback

[hansharhoff]

When not iscomplete occurs in the callback the current implementation returns None which generates server side error in user console. Instead we can raise PreventUpdate() to suppress this issue.

I also added some instructions to Contributing that would help newcomers get to an installed package quicker.
Added packaging to requirements as this is needed to import dash_uploader.

[fohrloop]

Hi! Thanks for the PR! I will look at it during the coming days! :) Does this PreventUpdate exist in all supported dash versions?

[fohrloop]

Nice catch! Did not previously see that packaging is required to import dash_uploader. However, requirements.txt is just meant just for developers. Correct place for requirements that should be installed automatically by pip when installing dash-uploader is the install_requires in the setup.py. Could you change the location of this requirement there?

#setup.py 
    install_requires=[
        "dash>=1.1.0",
    ],

[fohrloop]

Hi! Thanks for the PR! I will look at it during the coming days! :) Does this PreventUpdate exist in all supported dash versions?

I checked this and the PreventUpdate exists in dash 1.1.0, at least.

[fohrloop]

This would be ready to be merged once the packaging requirements is moved from requirements.txt to setup.py.

[hansharhoff]

Yes, done

[fohrloop]

Great! Thank you!