Themes: Add a return value to theme functions calling `locate_template()`:

* `get_header()`
* `get_footer()`
* `get_sidebar()`
* `get_template_part()`

These functions now return false if the template file could not be found, to allow for easier debugging.

Props tferry, sphakka, johnbillion, pento, davidbinda, desrosj, birgire, garrett-eclipse, williampatton, davidbaumwald, SergeyBiryukov.
Fixes #40969.
Built from https://develop.svn.wordpress.org/trunk@48209


git-svn-id: https://core.svn.wordpress.org/trunk@47978 1a063a9b-81f0-0310-95a4-ce76da25c4cd