check if update_period and frequency are valid

[ayrat555]

this feed (https://decrypt.co/rss) has invalid updatePeriod. because of this, parsing fails:

thread 'tokio-runtime-worker' panicked at 'called `Result::unwrap()` on an `Err` value: ()', /usr/local/cargo/registry/src/github.com-1ecc6299db9ec823/rss-2.0.0/src/extension/syndication.rs:170:40
stack backtrace:
   0: rust_begin_unwind
             at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/panicking.rs:143:14
   2: core::result::unwrap_failed
             at /rustc/7737e0b5c4103216d6fd8cf941b7ab9bdbaace7c/library/core/src/result.rs:1749:5
   3: rss::extension::syndication::SyndicationExtension::from_map
   4: rss::channel::Channel::from_xml
   5: rss::channel::Channel::read_from
   6: <el_monitorro::sync::reader::rss::RssReader as el_monitorro::sync::reader::ReadFeed>::read_from_bytes
   7: el_monitorro::sync::reader::validate_rss_url
   8: <el_monitorro::bot::commands::subscribe::Subscribe as el_monitorro::bot::commands::Command>::response
   9: el_monitorro::bot::commands::subscribe::Subscribe::execute
  10: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll

[andy128k]

Thanks!

[ayrat555]

would you mind releasing this change in a new release

[andy128k]

released in 2.0.1