add compile error for variable modified but not read

[andrewrk]

test "aoeu" {
    var x: i32 = 1234;
    x += 1;
}

This should be error: variable 'x' modified but not read. In order to silence the error, one could use the underscore identifier:

test "aoeu" {
    var x: i32 = 1234;
    x += 1;
    _ = x;
}

This second test is expected to pass.

Thanks to @mattsta for identifying this missing compile error in #2303.