Improve recovery on malformed format call

If a comma in a format call is replaced with a similar token, then we
emit an error and continue parsing, instead of stopping at this point.

Author: scrabsha

tests/ui/issue-3145.rs

@@ -1,3 +1,3 @@
 fn main() {
-    println!("{}" a); //~ERROR expected token: `,`
+    println!("{}" a); //~ERROR expected `,`, found `a`
 }

tests/ui/issue-3145.stderr

@@ -1,7 +1,7 @@
-error: expected token: `,`
+error: expected `,`, found `a`
   --> $DIR/issue-3145.rs:2:19
    |
-LL |     println!("{}" a); //~ERROR expected token: `,`
+LL |     println!("{}" a); //~ERROR expected `,`, found `a`
    |                   ^ expected `,`
 
 error: aborting due to previous error