Implementations of getInstance() return the wrong type

Problem/Motivation

Several implementations of getInstance() don't return the value described in the following documentation comment.

   * @return object|false
   *   A fully configured plugin instance. The interface of the plugin instance
   *   will depend on the plugin type. If no instance can be retrieved, FALSE
   *   will be returned.

FormatterPluginManager and WidgetPluginManager return NULL instead of FALSE.

Proposed resolution

Change the code to return FALSE as documented.