* load.c (scm_sys_try_load): don't check whether value returned

* load.c (scm_sys_try_load): don't check whether value returned
by scm_open_file is #f, it won't be.  Always return SCM_UNSPECIFIED.
Change the Scheme name from %try-load to primitive-load.

* error.c (scm_error): convert a NULL message to SCM_BOOL_F.
Can avoid passing a message, allowing it to be derived in the
error handler (e.g., if we want to throw to the key both from
Scheme and C).