Updated error message

I did something stupid here and forget to add `$set` operator to my update query. Before I used simple-schema I'd catch such errors pretty fast, but now I get the extremely cryptic error message. I had completely no idea what is the modifier option, where was it set, what are "validation object keys" and what does the "operator" mean here. Once I debugged all of this and find the cause I realized my mistake and what all those words mean, but it cost me over an hour. This could easily be avoided by more user-facing and less code-facing error message.