Improve localization for `WP_Date_Query` error strings.

* Don't create an extra variable for the string when it's only used once.
* Avoid using `<code>` HTML tags inside of translatable string.
* Improve data types and escaping on sprintfed values.

Props dd32.
Fixes #25834.
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git-svn-id: http://core.svn.wordpress.org/trunk@30299 1a063a9b-81f0-0310-95a4-ce76da25c4cd