all: change optional to result of io (#16075)

vlib/io/writer.v

@@ -3,11 +3,11 @@ module io
 // Writer represents a stream of data that can be wrote to
 pub interface Writer {
 mut:
-	write(buf []u8) ?int
+	write(buf []u8) !int
 }
 
 // RandomWriter represents a stream of data that can be wrote to
 // at a random pos
 pub interface RandomWriter {
-	write_to(pos u64, buf []u8) ?int
+	write_to(pos u64, buf []u8) !int
 }