[Diagnostics] Better error message for conversion of fun to System.Func

[0x53A]

The general rule of thumb when interoping with C# is that methods in C# are tupled in F#... except when you try to create a new System.Func.

Example:

open System

// Error:
// [FS0001] Type mismatch. Expecting a
//    'string -> int -> string * int'    
// but given a
//    ''a * 'b -> 'a * 'b'    
//The type 'string' does not match the type ''a * 'b'
let func : Func<string, int, string*int> =
   let f (s,i) = (s,i)
   Func<string, int, string*int>(f)

// Ok
let func : Func<string, int, string*int> =
   let f s i = (s,i)
   Func<string, int, string*int>(f)

In the spirit of the "elmish" error message improvements a few months ago, it would be awesome if the compiler could detect this case and give explicit instructions how to resolve this.