The original fix for Bug#6106 switched from signalling `search-failed'
to `user-error'. However, this breaks incremental searching over
multiple nodes because the isearch code doesn't expect a `user-error'.
* src/search.c (syms_of_search): New error, `user-search-failed',
with `user-error' and `search-failed' as parents.
* doc/lispref/errors.texi (Standard Errors): Document it.
* etc/NEWS: Announce it.
* lisp/info.el (Info-search): Use it instead of `user-error' so that
isearch will handle failed searches correctly.
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