Improve error message for undeclared class property initialization

Flow requires that class properties be declared before they can be initialized in a constructor. This behavior is reasonable, but the error message should guide the user to the solution:

``` javascript
/* @flow */

class Foo {
constructor() {
this.bar = "baz";
}
}
```

```
test.js|5 col 5 error| assignment of property bar
|| Property not found in
test.js|3 col 7 error| Foo
||
```

A better error message would direct the user to declare the type of `bar`, as follows:

``` javascript
/* @flow */

class Foo {
bar: string;
constructor() {
this.bar = "baz";
}
}
```

```
|| No errors!
```

A better message might be "Property must be declared in class."