Build/Test: Die with an error if populate_network fails

If you are missing WP_TESTS_EMAIL, populate_network will fail and it can be hard to debug. As populate_network can return a wp_error object, we can detect that and display the error to a user.

See: https://github.com/WordPress/gutenberg/pull/22613
Fixes: #50251
Props: TimothyBlynJacobs

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git-svn-id: https://core.svn.wordpress.org/trunk@47678 1a063a9b-81f0-0310-95a4-ce76da25c4cd