Posts, Post Types: Clarify some "Error in deleting" messages, use more...

Posts, Post Types: Clarify some "Error in deleting" messages, use more specific strings for attachments.

Props dingo_d.
See #50546.
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git-svn-id: https://core.svn.wordpress.org/trunk@48081 1a063a9b-81f0-0310-95a4-ce76da25c4cd