Improve error reporting: property syntax can't be remembered

# What

When I try to define a class with getters/setters (and I forget how this is supposed to be defined):

```fsharp
type Foo() =
member __.Prop : int with get,set
```

The compiler gives

> Error: A type definition requires one or more members or other declarations. If you intend to define an empty class, struct or interface, then use 'type ... = class end', 'interface end' or 'struct end'.

# Why

The syntax for some members (not only properties) can be difficult to remember when not used frequently (modules and functions are fine, but some frameworks insist on having properties, instance methods, etc.).

# How

In my case, there is "with get,set" which is a clue I want a property, although I should have typed

```fsharp
type Foo() =
member val Prop = 0 with get, set
```

I wish the compiler could help me out without resorting to google search.

related #1103