limit and delimit available fields in note

Also, don't show the note if no fields are available (usually due to
privacy).

src/test/compile-fail/E0559.rs

@@ -16,5 +16,5 @@ fn main() {
     let s = Field::Fool { joke: 0 };
     //~^ ERROR E0559
     //~| NOTE `Field::Fool` does not have this field
-    //~| NOTE available fields are: x
+    //~| NOTE available fields are: `x`
 }

src/test/compile-fail/E0560.rs

@@ -16,5 +16,5 @@ fn main() {
     let s = Simba { mother: 1, father: 0 };
     //~^ ERROR E0560
     //~| NOTE `Simba` does not have this field
-    //~| NOTE available fields are: mother
+    //~| NOTE available fields are: `mother`
 }

src/test/compile-fail/issue-19922.rs

@@ -16,5 +16,5 @@ fn main() {
     let homura = Homura::Akemi { kaname: () };
     //~^ ERROR variant `Homura::Akemi` has no field named `kaname`
     //~| NOTE `Homura::Akemi` does not have this field
-    //~| NOTE available fields are: madoka
+    //~| NOTE available fields are: `madoka`
 }

src/test/compile-fail/numeric-fields.rs

@@ -14,7 +14,7 @@ fn main() {
     let s = S{0b1: 10, 0: 11};
     //~^ ERROR struct `S` has no field named `0b1`
     //~| NOTE `S` does not have this field
-    //~| NOTE available fields are: 0, 1
+    //~| NOTE available fields are: `0`, `1`
     match s {
         S{0: a, 0x1: b, ..} => {}
         //~^ ERROR does not have a field named `0x1`

src/test/compile-fail/struct-fields-too-many.rs

@@ -18,6 +18,6 @@ fn main() {
         bar: 0
         //~^ ERROR struct `BuildData` has no field named `bar`
         //~| NOTE `BuildData` does not have this field
-        //~| NOTE available fields are: foo
+        //~| NOTE available fields are: `foo`
     };
 }

src/test/compile-fail/suggest-private-fields.rs

@@ -28,7 +28,7 @@ fn main () {
         bb: 20,
         //~^ ERROR struct `xc::B` has no field named `bb`
         //~| NOTE `xc::B` does not have this field
-        //~| NOTE available fields are: a
+        //~| NOTE available fields are: `a`
     };
     // local crate struct
     let l = A {

src/test/compile-fail/union/union-fields.rs

@@ -20,7 +20,7 @@ fn main() {
     let u = U { a: 0, b: 1, c: 2 }; //~ ERROR union expressions should have exactly one field
                                     //~^ ERROR union `U` has no field named `c`
                                     //~| NOTE `U` does not have this field
-                                    //~| NOTE available fields are: a, b
+                                    //~| NOTE available fields are: `a`, `b`
     let u = U { ..u }; //~ ERROR union expressions should have exactly one field
                        //~^ ERROR functional record update syntax requires a struct