Reduce nesting in externalfiles implementation

Utilize `?` instead of and_then/map operators

src/librustdoc/externalfiles.rs

@@ -25,34 +25,20 @@ impl ExternalHtml {
                 id_map: &mut IdMap, edition: Edition, playground: &Option<Playground>)
             -> Option<ExternalHtml> {
         let codes = ErrorCodes::from(UnstableFeatures::from_environment().is_nightly_build());
-        load_external_files(in_header, diag)
-            .and_then(|ih|
-                load_external_files(before_content, diag)
-                    .map(|bc| (ih, bc))
-            )
-            .and_then(|(ih, bc)|
-                load_external_files(md_before_content, diag)
-                    .map(|m_bc| (ih,
-                            format!("{}{}", bc, Markdown(&m_bc, &[], id_map,
-                                    codes, edition, playground).to_string())))
-            )
-            .and_then(|(ih, bc)|
-                load_external_files(after_content, diag)
-                    .map(|ac| (ih, bc, ac))
-            )
-            .and_then(|(ih, bc, ac)|
-                load_external_files(md_after_content, diag)
-                    .map(|m_ac| (ih, bc,
-                            format!("{}{}", ac, Markdown(&m_ac, &[], id_map,
-                                    codes, edition, playground).to_string())))
-            )
-            .map(|(ih, bc, ac)|
-                ExternalHtml {
-                    in_header: ih,
-                    before_content: bc,
-                    after_content: ac,
-                }
-            )
+        let ih = load_external_files(in_header, diag)?;
+        let bc = load_external_files(before_content, diag)?;
+        let m_bc = load_external_files(md_before_content, diag)?;
+        let bc = format!("{}{}", bc, Markdown(&m_bc, &[], id_map,
+                                    codes, edition, playground).to_string());
+        let ac = load_external_files(after_content, diag)?;
+        let m_ac = load_external_files(md_after_content, diag)?;
+        let ac = format!("{}{}", ac, Markdown(&m_ac, &[], id_map,
+                                    codes, edition, playground).to_string());
+        Some(ExternalHtml {
+            in_header: ih,
+            before_content: bc,
+            after_content: ac,
+        })
     }
 }