Improve error message of MissingExtension rejections (#72)

Now includes the name of missing type.

src/extract/mod.rs

@@ -831,7 +831,12 @@ where
             .extensions()
             .ok_or(ExtensionsAlreadyExtracted)?
             .get::<T>()
-            .ok_or(MissingExtension)
+            .ok_or_else(|| {
+                MissingExtension::from_err(format!(
+                    "Extension of type `{}` was not found. Perhaps you forgot to add it?",
+                    std::any::type_name::<T>()
+                ))
+            })
             .map(|x| x.clone())?;
 
         Ok(Extension(value))

src/extract/rejection.rs

@@ -66,7 +66,7 @@ define_rejection! {
     #[body = "Missing request extension"]
     /// Rejection type for [`Extension`](super::Extension) if an expected
     /// request extension was not found.
-    pub struct MissingExtension;
+    pub struct MissingExtension(BoxError);
 }
 
 define_rejection! {