PathError/SyntaxError implement the error interface (#758)

Signed-off-by: Talons.Lee <[email protected]>

eBook/13.1.md

@@ -70,7 +70,7 @@ type PathError struct {
 	Err error  // Returned by the system call.
 }
 
-func (e *PathError) String() string {
+func (e *PathError) Error() string {
 	return e.Op + " " + e.Path + ": "+ e.Err.Error()
 }
 ```
@@ -107,7 +107,7 @@ type SyntaxError struct {
 	Offset int64
 }
 
-func (e *SyntaxError) String() string { return e.msg }
+func (e *SyntaxError) Error() string { return e.msg }
 ```
 
 在调用代码中你可以像这样用类型断言测试错误是不是上面的类型：