Improve error message for using floats in contracts

[Bargsteen]

Description

Floating-point numbers are not supported in smart contracts, but if you don't know that and you try to use them (or you use a library that uses floats), the error message when building with cargo concordium is very cryptic.

Steps to Reproduce

1. Use floating-point numbers in your contract.
   Make sure that the compiler doesn't optimize the floats away.
   Something like this worked for me with 1.53.0:

   fn g() {
       if f(f(1.)) < 2. {
           println!("hello");
       }
   }
   
   #[inline(never)]
   fn f(x: f64) -> f64 {
       if x < 1. {
           x + 1.
       } else {
           x + 2.
       }
   }

2. Compile the contract with cargo concordium build

Expected Result

An error message saying: Floating point numbers are not allowed.

Actual Result

Error: Could not build smart contract.

Caused by:
    0: Could not validate resulting smart contract module as a V1 contract.
    1: Unknown value type byte 0x7c

(0x7c is the encoding of the f64 type in https://webassembly.github.io/spec/core/binary/types.html#number-types.)

Versions

• Software Version: cargo concordium v. 2.0.0
• OS: macOS

[abizjak]

Indeed. Although this will be substantially easier to do after #203 since right now errors are not easy to match on.