As discussed on IRC, it would be important to have the possibility of specifying a return value when an error is thrown.

The GIO stream APIs (read and write) expect '-1' to be returned along with the GError.

http://valadoc.org/#!api=gio-2.0/GLib.OutputStream.write

A CCode annotation would be necessary for functions throwing errors with a C expression:

    [CCode (return_value_on_error = "-1")]
    public override ssize_t write (uint8[] buffer) throws IOError {
        throw new IOError.FAILED ("");
    }

That would target C code, because a Vala caller would not be able to obtain the return value.

I might be interesting to consider the possibility of obtaining the return value in a try {} catch construct.