Make DNS-error-payload just contain a string.

Having `DNS-error-payload` record contain a string field for `rcode`
while a `u16` field for `info-code` is surprising. If appplications
will be expected to know the IANA values of the `u16` [INFO-CODE value],
it would make sense for them to know the `u16` for the [RCODE value] too.
But if they won't, then they likely don't have any use for either `u16`
value, and just want a simple string.

I think wasi-http could plausibly go either way: Provide `u16` values
for both `rcode` and `info-code`, or neither, and instead just provide
a `string`.

In this PR, I propose to just provide a `string`. My guess is that most
applications in scope here don't need precise DNS error code
information and basically just need a way to report that "it was DNS".
And, not all host resolver libraries provide error information that
includes RCODE and INFO-CODE, for example [Rust's `ToSocketAddrs` trait]
or [POSIX `getaddrinfo`].

Fixed #184.

[RCODE value]: https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-parameters-6
[INFO-CODE value]: https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#extended-dns-error-codes
[Rust's `ToSocketAddrs` trait]: https://doc.rust-lang.org/stable/std/net/trait.ToSocketAddrs.html
[POSIX `getaddrinfo`]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/freeaddrinfo.html

Author: sunfishcode

wit-0.3.0-draft/types.wit

@@ -75,8 +75,10 @@ interface types {
 
   /// Defines the case payload type for `DNS-error` above:
   record DNS-error-payload {
-    rcode: option<string>,
-    info-code: option<u16>
+    /// An indication of the reason for the DNS error, if known.
+    ///
+    /// This string is meant only for human consumption, and not for machine parsing.
+    reason: option<string>,
   }
 
   /// Defines the case payload type for `TLS-alert-received` above: