io: migrate the Reader API to Result instead of Option (#15229)

vlib/io/reader.v

@@ -1,13 +1,32 @@
 module io
 
+/// Eof error means that we reach the end of the stream.
+pub struct Eof {
+	Error
+}
+
+// NotExpected is a generic error that means that we receave a not expecte error.
+pub struct NotExpected {
+	cause string
+	code  int
+}
+
+fn (err NotExpected) msg() string {
+	return err.cause
+}
+
+fn (err NotExpected) code() int {
+	return err.code
+}
+
 // Reader represents a stream of data that can be read
 pub interface Reader {
 	// read reads up to buf.len bytes and places
 	// them into buf.
 	// A type that implements this should return
-	// `none` on end of stream (EOF) instead of just returning 0
+	// `io.Eof` on end of stream (EOF) instead of just returning 0
 mut:
-	read(mut buf []u8) ?int
+	read(mut buf []u8) !int
 }
 
 const (
@@ -50,7 +69,7 @@ pub fn read_any(mut r Reader) ?[]u8 {
 	mut b := []u8{len: io.read_all_len}
 	mut read := 0
 	for {
-		new_read := r.read(mut b[read..]) or { break }
+		new_read := r.read(mut b[read..]) or { return none }
 		read += new_read
 		if new_read == 0 {
 			break