Clarify @main error message for invalid return type

[huangyxi]

When the @main function returns a value that is not nothing or convertible to Cint, the current error message may be a bit unclear or unhelpful.

For example, running the script below results in an error message that doesn’t clearly point to the underlying cause:

#!/usr/bin/env julia

# Lengthy, potentially flawed code

function @main(args)
	# Lengthy, potentially flawed code
	[] # Valid in regular functions, but not in `@main`
end

In this case, returning an empty array ([]) is perfectly fine in a normal function, but it’s not allowed in a @main context. The current error message doesn’t clearly emphasize this, which can make it tricky for users to understand and resolve the issue:

ERROR: MethodError: no method matching Int32(::Vector{Any})
The type `Int32` exists, but no method is defined for this combination of argument types when trying to construct it.

Closest candidates are:
  Int32(::Float64)
   @ Base float.jl:895
  Int32(::Float32)
   @ Base float.jl:919
  Int32(::Float16)
   @ Base float.jl:919
  ...

Stacktrace:
 [1] _start()
   @ Base ./client.jl:568

[huangyxi]

This PR would fix #56328.