matchers: `to_raise` as an alias for `to_error`.

Avoid spurious matches for capitalised error messages from
Slingshot sanity checks.
* specs/matchers_spec.yaml (raise): Specify behaviours of new
alias.
* lib/specl/matchers.lua (error): Rename from this...
(raise): ...to this.
(error): Reinstate as an alias for the new `raise` matcher.
* specs/inprocess_spec.yaml, specs/shell_spec.yaml: Use `raise`
matcher by default.
* NEWS: Update.

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