Define new functions to manipulate error descriptors and
add support for `signal` to *re*signal a previous error.
* src/eval.c (Fsignal): Make the second arg optional and document
the possibility of passing a whole error descriptor to re-signal it.
(signal_or_quit): Fix a few corner case issues when
DATA is `nil` and ERROR_SYMBOL is an error descriptor.
* lisp/subr.el (error-type-p, error--p, error-type, error-data)
(error-has-type-p, error-slot-value): New function.
* doc/lispref/control.texi (Handling Errors): Prefer "error descriptor"
to "error description". Use the new single-arg call to `signal` to
re-throw an error.
Document `error-type`, `error-data` and `error-slot-value`.
(Error Symbols): Document the new functions `error-type-p` and
`error-has-type-p`.
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