[Error Messages] Unhelpful error when trying to augment a generic type with concrete type parameters

```F#
module X =

open System

// this compiles
type Tuple<'T1, 'T2> with
member x.I() = 1

// this fails with:
// error FS0010: Unexpected identifier in type name. Expected infix operator, quote symbol or other token.
type Tuple with
member x.I() = 1
```

### Fix

```
open System.Runtime.CompilerServices

[]
type TupleEx() =
[]
static member inline I1(self: Tuple) = 1
[]
static member inline I2(self: int*_) = 1

let x = (1, "")
let i1 = x.I1()
let i2 = x.I2()

```

It would be nice if the first code snippet failed with an explicit error message, something like

> generic types with concrete type parameters can not be extended. You may want to use explicit extension methods using the ``[]`` Attribute. See https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/type-extensions

Note: the second snippet fails in the released version, but compiles on master due to the tuple fixes by dsyme, thank you!