core: modernize the SwitchRoot() bus method a bit

core: modernize the SwitchRoot() bus method a bit

Let's more verbose error messages when validating the input parameters fails.
Also, call path_is_os_tree() properly, as it doesn't return a boolean, but
possibly also an error. Finally, check for the existance of the new init
process with chase_symlinks() to properly handle possible symlinks on the init
binary (which might actually be pretty likely).