os, syscall: fix Plan 9 build

R=rsc
CC=golang-dev
https://golang.org/cl/5330067

src/pkg/os/env_plan9.go

@@ -7,7 +7,7 @@
 package os
 
 import (
-	"error"
+	"errors"
 	"syscall"
 )
 

src/pkg/os/error_plan9.go

@@ -28,7 +28,7 @@ func NewSyscallError(syscall string, err syscall.Error) error {
 	if err == nil {
 		return nil
 	}
-	return &SyscallError{syscall, err.String()}
+	return &SyscallError{syscall, err.Error()}
 }
 
 var (

src/pkg/os/exec_plan9.go

@@ -5,6 +5,7 @@
 package os
 
 import (
+	"errors"
 	"runtime"
 	"syscall"
 )
@@ -47,7 +48,7 @@ func (note Plan9Note) String() string {
 
 func (p *Process) Signal(sig Signal) error {
 	if p.done {
-		return NewError("os: process already finished")
+		return errors.New("os: process already finished")
 	}
 
 	f, e := OpenFile("/proc/"+itoa(p.Pid)+"/note", O_WRONLY, 0)

src/pkg/syscall/syscall_plan9.go

@@ -19,13 +19,13 @@ const ImplementsGetwd = true
 
 // An Error can represent any printable error condition.
 type Error interface {
-	String() string
+	error
 }
 
 // ErrorString implements Error's String method by returning itself.
 type ErrorString string
 
-func (e ErrorString) String() string { return string(e) }
+func (e ErrorString) Error() string { return string(e) }
 
 // NewError converts s to an ErrorString, which satisfies the Error interface.
 func NewError(s string) Error { return ErrorString(s) }