Indicate in the UI when there's a parsing error

[nfischer]

Right now it's a parse error to write something like:

echo hello world && echo bye

The first & is interpreted as "run in background job" (obviously the wrong interpretation) and the rest of the line causes a parse error (so additional lines don't get transpiled). Errors like this should be indicated more clearly (ideally, they would be inline as code-mirror widgets). I have a similar solution at rainbows-lang.