Make `GuildChannel::send_message` work in threads (#1449)
Make `GuildChannel::send_message` work in threads (#1449)

When the cache feature is enabled, `GuildChannel::send_message` invokes `utils::user_has_perms`, which tries to look up the thread in the channels cache. Threads are often not in cache, so it fails and throws a `model::Error::ChannelNotFound`. `send_message` lazily propagates any error, so the cache miss brings down the whole `send_message` invocation.

This commit fixes `send_message` to not abort the invocation if the sanity check throws an error.
Flat/serenity,acdenisSK/serenity