If a custom plugin or theme exists in the `build/wp-content` directory with a sourcemap, the build` script is currently returning a warning and failing.
This improves the `verify:source-maps` task in Grunt to ignore directories and files that do not belong to WordPress Core.
Props ryelle, afragen, johnbillion.
Fixes#52689.
Built from https://develop.svn.wordpress.org/trunk@51173
git-svn-id: http://core.svn.wordpress.org/trunk@50782 1a063a9b-81f0-0310-95a4-ce76da25c4cd