os: homogenize error checks on Plan 9

Remove leading space at the beginning of error strings,
so the strings are consistent between isExist, isNotExist
and isPermission functions.

Here is a list of error strings returned on the most common
file servers on Plan 9:

     match                     cwfs                      fossil                   ramfs

"exists"            "create/wstat -- file exists"  "file already exists"    "file exists"
"is a directory"                                   "is a directory"         "file is a directory"

"does not exist"                                   "file does not exist"    "file does not exist"
"not found"         "directory entry not found"
"has been removed"                                 "file has been removed"

"permission denied" "access permission denied"     "permission denied"      "permission denied"

"no parent" is an error returned by lib9p when removing a file without parent.

Change-Id: I2362ed4b6730b8bec7a707a1052bd1ad8921cd97
Reviewed-on: https://go-review.googlesource.com/89315Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/os/error_plan9.go

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