400 if account already assocaited (#1048)

internal/errors/code_test.go

@@ -97,6 +97,11 @@ func TestCode_Both_String_Info(t *testing.T) {
 			c:    TooShort,
 			want: TooShort,
 		},
+		{
+			name: "AccountAlreadyAssociated",
+			c:    AccountAlreadyAssociated,
+			want: AccountAlreadyAssociated,
+		},
 		{
 			name: "InternalError",
 			c:    Internal,