added error text for multiple associations

public/language/en-GB/error.json

@@ -159,6 +159,7 @@
 	"wrong-login-type-email": "Please use your email to login",
 	"wrong-login-type-username": "Please use your username to login",
 	"sso-registration-disabled": "Registration has been disabled for %1 accounts, please register with an email address first",
+	"sso-multiple-association": "You cannot associate multiple accounts from this service to your NodeBB account. Please dissociate your existing account and try again.",
 
 	"invite-maximum-met": "You have invited the maximum amount of people (%1 out of %2).",