Rollup merge of #35620 - cvubrugier:master, r=Manishearth

book: fix the hidden find() functions in error-handling.md

The hidden find() functions always returns None. Consequently, one of the
examples using find() prints "No file extension found" instead of
"File extension: rs" which is the expected output.

This patch fixes the issue by implementing find() with std::str::find().

Signed-off-by: Christophe Vu-Brugier <cvubrugier@fastmail.fm>