os: use error, io.EOF

R=r
CC=golang-dev
https://golang.org/cl/5298073
diff --git a/src/pkg/os/error.go b/src/pkg/os/error.go
index b4511dd..c3dd06f 100644
--- a/src/pkg/os/error.go
+++ b/src/pkg/os/error.go
@@ -4,28 +4,11 @@
 
 package os
 
-// An Error can represent any printable error condition.
-type Error interface {
-	String() string
-}
-
-// // errorString is a helper type used by NewError.
-type errorString string
-
-func (e errorString) String() string { return string(e) }
-
-// Note: If the name of the function NewError changes,
-// pkg/go/doc/doc.go should be adjusted since it hardwires
-// this name in a heuristic.
-
-// // NewError returns a new error with error.String() == s.
-func NewError(s string) Error { return errorString(s) }
-
 // PathError records an error and the operation and file path that caused it.
 type PathError struct {
-	Op    string
-	Path  string
-	Error Error
+	Op   string
+	Path string
+	Err  error
 }
 
-func (e *PathError) String() string { return e.Op + " " + e.Path + ": " + e.Error.String() }
+func (e *PathError) Error() string { return e.Op + " " + e.Path + ": " + e.Err.Error() }