Text Changes: Merge some duplicate strings with the same meaning in error...

Text Changes: Merge some duplicate strings with the same meaning in error messages, adjust some other strings for consistency and accuracy.

Props ramiy, SergeyBiryukov.
Fixes #38808.
Built from https://develop.svn.wordpress.org/trunk@39278


git-svn-id: https://core.svn.wordpress.org/trunk@39218 1a063a9b-81f0-0310-95a4-ce76da25c4cd