claude: complete Err -> Error module rename across call sites

Follow up to the module rename: update the remaining callers that
still referenced [Err] (library [claude.ml{,i}], [client.ml], the test
driver [test.ml]), and fix one stray [^ e] string concatenation in
hermest's CLI that needed [Json.Error.to_string e] now that
[Json.of_string] yields a structured error.