publish-all script should bail early when there are errors

[chriddyp]

I tried publishing but twine wasn't installed. Instead of bailing early, the script continued to the rest of the steps. The fact that twine wasn't installed was buried in the console output (sh: twine: command not found)

[...]
copying dash_core_components.egg-info/PKG-INFO -> dash_core_components-0.39.0rc1/dash_core_components.egg-info
copying dash_core_components.egg-info/SOURCES.txt -> dash_core_components-0.39.0rc1/dash_core_components.egg-info
copying dash_core_components.egg-info/dependency_links.txt -> dash_core_components-0.39.0rc1/dash_core_components.egg-info
copying dash_core_components.egg-info/requires.txt -> dash_core_components-0.39.0rc1/dash_core_components.egg-info
copying dash_core_components.egg-info/top_level.txt -> dash_core_components-0.39.0rc1/dash_core_components.egg-info
copying test/test_dash_import.py -> dash_core_components-0.39.0rc1/test
copying test/test_integration.py -> dash_core_components-0.39.0rc1/test
Writing dash_core_components-0.39.0rc1/setup.cfg
creating dist
Creating tar archive
removing 'dash_core_components-0.39.0rc1' (and everything under it)
sh: twine: command not found
Counting objects: 5, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (5/5), done.
Writing objects: 100% (5/5), 1.12 KiB | 1.12 MiB/s, done.
Total 5 (delta 3), reused 0 (delta 0)
remote: Resolving deltas: 100% (3/3), completed with 3 local objects.
To https://github.com/plotly/dash-core-components
 * [new tag]         v0.39.0rc1 -> v0.39.0rc1