Add return constants package, and common message sender functions

Trying to resolve as much as possible ``gometalinter`` issues.

lib/constants/constants.go

@@ -0,0 +1,15 @@
+// i2_bot – Instinct PokememBro Bot
+// Copyright (c) 2017-2018 Vladimir "fat0troll" Hodakov
+
+package constants
+
+const (
+	// BotError is return value, if something went wrong with bot's internal data handling
+	BotError = "bot_error"
+	// UserRequestFailed is return value, if something went wrong with user input
+	UserRequestFailed = "fail"
+	// UserRequestSuccess is success return value
+	UserRequestSuccess = "ok"
+	// UserRequestForbidden is return value, if user can't perform some action
+	UserRequestForbidden = "forbidden"
+)