refactor: use to_raise matcher alias consistently.

Slingshot sanity checks flag strings with an initial capital
following 'error' as bad style.  Rather than switch off that
check, use a matcher alias.
* build-aux/sanity-cfg.mk (sc_error_message_uppercase): Remove
spec-files from regexp.
specs/base_spec.yaml, specs/container_spec.yaml,
specs/debug_spec.yaml, specs/functional_spec.yaml,
specs/io_spec.yaml, specs/list_spec.yaml, specs/math_spec.yaml,
specs/optparse_spec.yaml, specs/package_spec.yaml,
specs/std_spec.yaml, specs/string_spec.yaml,
specs/table_spec.yaml, specs/tree_spec.yaml,
specs/vector_spec.yaml: s/to_error/to_raise/
s/not_to_raise ()/not_to_raise "any error"/

Signed-off-by: Gary V. Vaughan <gary@gnu.org>