Make a better interface for informing users of syntax errors inline

right now the caret turns red. That might be sufficient, but it's probably not a strong enough indicator.

remember: we don't want the user to get overwhelmed because they keep making syntax errors while they're typing. We want to assist them, not shout errors at them loudly.

Start by locating the code that turns the caret red when the editor detects a syntax error. Review the existing inline error behavior and any related interface entry points; done should be a decided, non-overwhelming way to inform users of syntax errors while they type.

Written by the indexing model from the issue text.