[Vala] [BUG] Valac fails to emit throws on vapi generation

[tecywiz121 <tecywiz121 hotmail com>]

Fairly simple bug, when compiling the following code, valac doesn't
output the proper throws keyword on the delegate.

//compile with valac --vapi=throws.vapi throws.vala
public delegate void TestFunc() throws Error;
void main() {}

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I get the following vapi file:

[CCode (cheader_filename = "throws.h")]
public delegate void TestFunc ();