os: handle ' is a directory' error as IsExist on Plan 9

This error is returned by os.Mkdir when the directory already exists.

This change fixes some upspin tests.

Change-Id: I9ad5aefebb32dff577726d537b4f3826d79868eb
Reviewed-on: https://go-review.googlesource.com/88656Reviewed-by: David du Colombier <0intro@gmail.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/error_plan9.go

@@ -5,7 +5,7 @@
 package os
 
 func isExist(err error) bool {
-	return checkErrMessageContent(err, " exists")
+	return checkErrMessageContent(err, " exists", " is a directory")
 }
 
 func isNotExist(err error) bool {