Check if device already claimed

Client can claim only unclaimed device.
Cloud can claim any device.
Claim with kNone is not allowed.

BUG=25766815, 26143922

Change-Id: Id92168b7f7c290509e672a659f09b7d06af37b76
Reviewed-on: https://weave-review.googlesource.com/1960
Reviewed-by: Vitaly Buka <vitalybuka@google.com>
diff --git a/src/privet/constants.cc b/src/privet/constants.cc
index 3cc3e4a..e1962bd 100644
--- a/src/privet/constants.cc
+++ b/src/privet/constants.cc
@@ -30,6 +30,7 @@
 const char kInvalidPassphrase[] = "invalidPassphrase";
 const char kNotFound[] = "notFound";
 const char kNotImplemented[] = "notImplemented";
+const char kAlreadyClaimed[] = "alreadyClaimed";
 
 }  // namespace errors
 }  // namespace privet