Replace assert!()s with proper errors in parsers.

src/jingle.rs

@@ -209,7 +209,10 @@ pub struct Jingle {
 }
 
 pub fn parse_jingle(root: &Element) -> Result<Jingle, Error> {
-    assert!(root.is("jingle", JINGLE_NS));
+    if !root.is("jingle", JINGLE_NS) {
+        return Err(Error::ParseError("This is not a Jingle element.")),
+    }
+
     let mut contents: Vec<Content> = vec!();
 
     let action = root.attr("action")