Expose Tika http status code in errors returned by client methods

[tomyl]

For users of tika.Client it can be useful to be able to differentiate between intermittent errors (http status code 500) and content related errors (e.g. 415 and 422) however currently the client methods just return an opaque error string.

This PR exposes the http status code in the errors returned by the client methods. Example usage:

func doStuff(input io.Reader, tikaURL string) error {
    client := tika.NewClient(nil, tikaURL)
    s, err := client.Parse(context.Background(), input)
    if isUnsupportedFileFormat(err) {
        return nil
    }
    if err != nil {
        return err
    }
   ...
}

func isUnsupportedFileFormat(err error) bool {
    var tikaErr tika.ClientError

    if errors.As(err, &tikaErr) {
        switch tikaErr.StatusCode {
        // Password protected documents yield StatusUnprocessableEntity
        case http.StatusUnsupportedMediaType, http.StatusUnprocessableEntity:
            return true
        default:
            return false
        }
    }

    return false
}

I considered going with type ClientError int but I thought that a struct is more future proof. Perhaps it could contain information about the exact Tika exception in the future.

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[tomyl]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[tbpg]

Thanks! I left a couple minor comments. But, this looks good!

[tbpg]

Sorry this fell off my radar. Looks good! Thanks!