Improve error reporting: Trying to pass arguments to a simple value gives confusing error.

## What
This code leads to the following error:

```fsharp
let x = 5
x 10
^
```
```
error FS0003: This value is not a function and cannot be applied.
```

## Why
Beginners to the language try to read this error and 9/10 times don't understand what it means.

## How
Consider replacing the error message with something like:

```
"x" is a simple value of type "int", and not a function. It does not take in any arguments, but you are trying to pass the value "10" as an argument to it.
```