Fix missing script file due to changes in React build process

[nylen]

Currently, Gutenberg will fail to load with the following error:

It may be necessary to rm vendor/*.js in order to see this error, because by default these vendor JS files are cached for one day.

This failure occurs because we attempt to load https://unpkg.com/react-dom@next/umd/react-dom-server.production.min.js, which currently redirects to https://unpkg.com/[email protected]/umd/react-dom-server.production.min.js, which is not found.

As seen in the directory listing at https://unpkg.com/[email protected]/umd/ and the related React change (facebook/react#10362), the correct path to this file now includes a .browser suffix.

The first commit in this PR adds error handling to detect this condition (and any other such failures) when building a plugin zip file:

$ bin/build-plugin-zip.sh 
WARNING: You should probably be running this script against the
         'master' branch (current: 'fix/build-error-react-dom-server')

https://unpkg.com/react@next/umd/react.production.min.js
 > vendor/react.min.c7397a88.js ... done!
https://unpkg.com/react-dom@next/umd/react-dom.production.min.js
 > vendor/react-dom.min.912dc6cf.js ... done!
https://unpkg.com/react-dom@next/umd/react-dom-server.production.min.js
 > vendor/react-dom-server.min.538e07c8.js ... error - HTTP 404

The second commit fixes the load issue (and the plugin zip build) by fixing the path.

[nylen]

Merging this to fix the build.