* numbers.c: use SCM_NUM_OVERFLOW instead of scm_wta or ASSERT.

* numbers.c: use SCM_NUM_OVERFLOW instead of scm_wta or ASSERT.

* error.c, error.h: setup scm_num_overflow key.

* __scm.h: SCM_NUM_OVERFLOW: macro for reporting numerical overflow.
Remove definition of SCM_OVSCM_FLOW.

* fports.c (scm_open_file): use SCM_SYSERROR_M.

* __scm.h: SCM_SYSERROR_M: new macro for system errors with an
explicit message and args.

* error.c, error.h, __scm.h: change system_error_sym to
scm_system_error.

* error.c (system_error_sym): remove leading %% from the Scheme name
"%%system-error".

* __scm.h (SCM_SYSMISSING): Redefine using lgh_error.

* boot-9.scm: remove leading %% from references to '%%system-error.
(%%handle-system-error): don't pass all the thrown arguments when
aborting, just the key and subr.
Remove the code to "Install default handlers for built-in errors."
Remove the definition of the syserror procedure.
Associate 'numerical-overflow with default handler.