Better error message when the server crashes

[AlexKontorovich]

Now it just says "server crashed"; I think it would help if it could say something like "just reload this page".

[cauli]

Related comment: #134 (comment)

Maybe we could eliminate the server for compilation altogether by running Lean in the browser directly via WASM

[abentkamp]

Unfortunately, there is no wasm build of Lean. I am not sure if the lean2wasm repo you linked works at all, but I am pretty sure that it cannot build the entire Lean server. Search for wasm on the Lean zulip for more info on this.