Return partial String response even for invalid utf-8

[Arnavion]

Currently the generated code for String responses (Pod::read_namespaced_pod_log, etc) returns Ok(&buf[..valid_up_to]) in case of truncated utf-8, but Err in the case of invalid utf-8 anywhere in the buffer.

However, it's useful to return Ok(&buf[..valid_up_to]) even in the case of invalid utf-8, and only return the Err if valid_up_to is 0. This way the caller gets as much of the parsed response as possible, rather than a variable amount depending on how they happened to chunk their response buffer.