As with all dependency validation, true is returned on success, PEAR_Error on failure
for required dependencies (and the PEAR installer dependency is required). If the
options are specified, an array containing the error message will be returned instead.
This method retrieves the list of packages to be uninstalled from the PEAR_Installer object, and makes sure no dependencies with existing packages would be broken if the current package were to be uninstalled.
throws no exceptions thrown
This function can not be called statically.