error message on overloaded main function can be improved

Given the following code:
```
function main() { }

function main(n: U32) { }
```
This issues the following error message:
```
C:\dev\Cpp\Seatbelt\test.bs:1:1: main function must not be overloaded
```
This could be improved: The error should point to the second overload of main that has been found (instead of the start of the file).