(Info-isearch-initial-node): New internal variable.

(Info-isearch-initial-node): New internal variable.
(Info-search): Signal an error in isearch mode when search leaves
the initial node.  Signal an error when `bound' is non-nil and
nothing was found in the current subfile.
(Info-isearch-search): Remove `condition-case'.
(Info-isearch-wrap): Don't wrap when search failed during leaving
the initial node.  If `Info-isearch-search' is nil, wrap around
the current node.
(Info-isearch-start): New fun.
(Info-mode): Add buffer-local hook `Info-isearch-start' to
`isearch-mode-hook'.