add exception_on_error optional arg to primitive-load-path

add exception_on_error optional arg to primitive-load-path

* libguile/init.c (scm_load_startup_files): Use
  scm_c_primitive_load_path.

* libguile/load.c (scm_primitive_load_path): Add an optional arg,
  exception_on_error, which if #f will cause primitive-load-path to just
  return #f if no file is found. This is to help out the semantics of
  try-module-autoload. We can't just catch misc-error, because loading
  the file could raise any exception.
  (scm_c_primitive_load_path): Add the extra arg to
  scm_primitive_load_path.

* libguile/load.h: Adapt scm_primitive_load_path prototype.

* module/ice-9/boot-9.scm (try-module-autoload): Refactor slightly to be
  clearer.