Login and Registration: Introduce `lostpassword_errors` filter in...

Login and Registration: Introduce `lostpassword_errors` filter in `retrieve_password()` for errors encountered on a password reset request.

This complements the `registration_errors` filter in `register_new_user()`.

Props wpdo5ea, dilipbheda.
Fixes #49521.
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git-svn-id: https://core.svn.wordpress.org/trunk@47851 1a063a9b-81f0-0310-95a4-ce76da25c4cd