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.h b/src/privet/constants.h
index 0668879..2b001aa 100644
--- a/src/privet/constants.h
+++ b/src/privet/constants.h
@@ -32,6 +32,7 @@
 extern const char kInvalidPassphrase[];
 extern const char kNotFound[];
 extern const char kNotImplemented[];
+extern const char kAlreadyClaimed[];
 }  // namespace errors
 }  // namespace privet
 }  // namespace weave