The full error message is missing so most likely no one saw that error unless they had the bottom of their monitor blocked in some way.

I'm on 2014 and lower so not sure if it's changed with 2016 but I would expect the SETUSER error instead.
If the full error was because the account you are using can't be found, can't be impersonated or you don't have permissions...however that full error goes then yes. Execute as User would require the user to explicitly be in the database.

Sue