Error documentation improvement

[MikaelMayer]

Consider this program (credits to @robin-aws )

 class C {
   var x: nat
   constructor(x: nat) 
     ensures this.x == x
   {
     this.x := x;
   }
 }
 
 method Foo() {
   var c := new C(42);
   assert c.x == 42;
   assert fresh(c);
 
   assert IsFresh(c); // Error: argument ('c') might not be allocated in the two-state function's previous state
 }
 
 twostate predicate IsFresh(c: C) {
   fresh(c)
 }

We should change the error message to

Error: argument ('c') might not be allocated in the two-state function's previous state. Did you miss 'new' before the parameter declaration, like 'new c: C' ? It allows unallocated parameters explicitly.