doc: Update agent API with possible errors

index ef80499e..5200dfb5 100644
@@ -43,6 +43,7 @@ Methods		void Release()
 			a network and passphrase is required.
 
 			Possible Errors: net.connman.iwd.Agent.Error.Canceled
+					 net.connman.iwd.Agent.Error.Failed
 
 		string RequestPrivateKeyPassphrase(object network)
 

@@ -52,6 +53,7 @@ Methods		void Release()
 			obtain that private key's encryption passphrase.
 
 			Possible Errors: net.connman.iwd.Agent.Error.Canceled
+					 net.connman.iwd.Agent.Error.Failed
 
 		(string, string) RequestUserNameAndPassword(object network)
 

@@ -60,6 +62,7 @@ Methods		void Release()
 			user name and password.
 
 			Possible Errors: net.connman.iwd.Agent.Error.Canceled
+					 net.connman.iwd.Agent.Error.Failed
 
 		string RequestUserPassword(object network, string user)
 

@@ -69,6 +72,7 @@ Methods		void Release()
 			in the parameter.
 
 			Possible Errors: net.connman.iwd.Agent.Error.Canceled
+					 net.connman.iwd.Agent.Error.Failed
 
 		void Cancel(string reason)