fix incorrect injection of error messages
fix incorrect injection of error messages

The error messages are not formatted as HTML and need to be injected
with textContent rather than innerHTML.

Thanks to Fabio Bonelli (@bfabio) for discovering this issue.

drlagos/mpisessions-playpen,Enamex/rust-playpen,Limvot/kraken-playpen,lastorset/rust-playpen,cogumbreiro/scribble-playpen,plietar/pony-playpen,Enamex/rust-playpen,plietar/pony-playpen,drlagos/unify-playpen,plietar/pony-playpen,epdtry/cse333-rust-slides,Limvot/kraken-playpen,cogumbreiro/scribble-playpen,rust-lang/rust-playpen,drlagos/mpisessions-playpen,lastorset/rust-playpen,Enamex/rust-playpen,chris-morgan/rust-playpen,rust-lang/rust-playpen,rust-lang/rust-playpen,tilpner/rust-playpen,cogumbreiro/scribble-playpen,Enamex/rust-playpen,cogumbreiro/sepi-playpen,cogumbreiro/sepi-playpen,drlagos/unify-playpen,chris-morgan/rust-playpen,chris-morgan/rust-playpen,chris-morgan/rust-playpen,epdtry/cse333-rust-slides,tilpner/rust-playpen,lastorset/rust-playpen,rust-lang/rust-playpen,tilpner/rust-playpen,tilpner/rust-playpen,plietar/pony-playpen,epdtry/cse333-rust-slides,epdtry/cse333-rust-slides,lastorset/rust-playpen,Limvot/kraken-playpen