Throw a `search-failed' derived error in Info search

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.

1 files changed, 6 insertions, 0 deletions

diff --git a/doc/lispref/errors.texi b/doc/lispref/errors.texi
index 2ec1a108ea9..1f67819c34e 100644
--- a/doc/lispref/errors.texi
+++ b/doc/lispref/errors.texi
@@ -186,6 +186,12 @@ The message is @samp{Undefined color}.  @xref{Color Names}.
 @item user-error
 The message is the empty string.  @xref{Signaling Errors}.
 
+@item user-search-failed
+This is like @samp{search-failed}, but doesn't trigger the debugger,
+like @samp{user-error}.  @xref{Signaling Errors}, and @xref{Searching
+and Matching}.  This is used for searching in Info files, @xref{Search
+Text,,,info,Info}.
+
 @item void-function
 The message is @samp{Symbol's function definition is void}.
 @xref{Function Cells}.