parsers: use Error type from xso

This is a large change and as such, it needs good motivation. Let me
remind you of the ultimate goal: we want a derive macro which allows us
to FromXml/IntoXml, and that derive macro should be usable from
`xmpp_parsers` and other crates.

For that, any code generated by the derive macro mustn't depend on any
code in the `xmpp_parsers` crate, because you cannot name the crate you
are in portably (`xmpp_parsers::..` wouldn't resolve within
`xmpp_parsers`, and `crate::..` would point at other crates if the macro
was used in other crates).

We also want to interoperate with code already implementing
`TryFrom<Element>` and `Into<Element>` on structs. This ultimately
requires that we have an error type which is shared by the two
implementations and that error type must be declared in the `xso` crate
to be usable by the macros.

Thus, we port the error type over to use the type declared in `xso`.

This changes the structure of the error type greatly; I do not think
that `xso` should have to know about all the different types we are
parsing there and they don't deserve special treatment. Wrapping them in
a `Box<dyn ..>` seems more appropriate.

parsers/src/pubsub/owner.rs

@@ -9,9 +9,9 @@ use crate::data_forms::DataForm;
 use crate::iq::{IqGetPayload, IqResultPayload, IqSetPayload};
 use crate::ns;
 use crate::pubsub::{AffiliationAttribute, NodeName, Subscription};
-use crate::util::error::Error;
 use crate::Element;
 use jid::Jid;
+use xso::error::{Error, FromElementError};
 
 generate_element!(
     /// A list of affiliations you have on a service, or on a node.
@@ -143,9 +143,9 @@ impl IqSetPayload for PubSubOwner {}
 impl IqResultPayload for PubSubOwner {}
 
 impl TryFrom<Element> for PubSubOwner {
-    type Error = Error;
+    type Error = FromElementError;
 
-    fn try_from(elem: Element) -> Result<PubSubOwner, Error> {
+    fn try_from(elem: Element) -> Result<PubSubOwner, FromElementError> {
         check_self!(elem, "pubsub", PUBSUB_OWNER);
         check_no_attributes!(elem, "pubsub");
 
@@ -153,17 +153,18 @@ impl TryFrom<Element> for PubSubOwner {
         for child in elem.children() {
             if child.is("configure", ns::PUBSUB_OWNER) {
                 if payload.is_some() {
-                    return Err(Error::ParseError(
+                    return Err(Error::Other(
                         "Payload is already defined in pubsub owner element.",
-                    ));
+                    )
+                    .into());
                 }
                 let configure = Configure::try_from(child.clone())?;
                 payload = Some(PubSubOwner::Configure(configure));
             } else {
-                return Err(Error::ParseError("Unknown child in pubsub element."));
+                return Err(Error::Other("Unknown child in pubsub element.").into());
             }
         }
-        payload.ok_or(Error::ParseError("No payload in pubsub element."))
+        payload.ok_or(Error::Other("No payload in pubsub element.").into())
     }
 }