libraries_get_path() can return bool

Problem/Motivation

In case the module is enabled, but the file is not where the libraries API module expects to find it, then libraries_get_path() returns false, which breaks _slim_select_get_path() signature

Steps to reproduce

enable libraries api module, make sure there is not slim select in its expected folders, apply the #slim_select to a multiple select, check the form page.

Proposed resolution

change from
add the result of libraries_get_path('slim-select') into a variable and check the result before returning

Remaining tasks

User interface changes

API changes

Data model changes