新增SystemError枚举类型，使得错误处理更清晰 (#205)

kernel/src/net/mod.rs

@@ -0,0 +1,9 @@
+use core::fmt::Debug;
+
+use crate::syscall::SystemError;
+
+pub trait Socket: Sync + Send + Debug {
+    fn read(&self, buf: &mut [u8]) -> Result<usize, SystemError>;
+
+    fn write(&self, buf: &[u8]) -> Result<usize, SystemError>;
+}