tamer: xir: Attribute error formatting/typo fixes

There were two problem errors: one showing "element element" and one showing
the value along with the name of the attribute.

The change for `<Attr as Display>::fmt` is debatable.  I'm going to do this
for now (only show `@name`) and adjust later if necessary.

I'll need to go use `crate::fmt` consistently in previously-existing format
strings at some point, too.

DEV-7145

tamer/src/xir/attr.rs

@@ -177,8 +177,17 @@ impl crate::parse::Object for Attr {}
 
 impl Display for Attr {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        // Do not display value since it can contain any information and
+        //   mess up formatted output.
+        // If we wish to display that information in the future,
+        //   then we ought to escape and elide it,
+        //   but we must furthermore make sure that it makes sense in all
+        //     contexts;
+        //       many diagnostic messages today expect that outputting an
+        //       attribute will output the name of that attribute and
+        //       nothing more.
         match self {
-            Self(key, value, _) => write!(f, "`@{key}=\"{value}\"`"),
+            Self(key, _value, _) => write!(f, "@{key}"),
         }
     }
 }

tamer/src/xir/parse/attr.rs

@@ -110,7 +110,7 @@ impl<S: AttrParseState> Display for AttrParseError<S> {
                 write!(
                     f,
                     "unexpected attribute `{attr}` for \
-                       element element `{ele_name}`"
+                       element `{ele_name}`"
                 )
             }