make the shift+enter in code editor warning clearer -- (was: "Mysterious error banner")

[novoselt]

I got "You can evaluate code in a file with the extension 'sagews' or 'ipynb'. Please create a Sage Worksheet or Jupyter notebook instead." as a red, i.e. error banner while editing a markdown file.

I have no idea why it has appeared, perhaps I've pressed some keyboard shortcut? If so, it would be helpful to see it in the banner "You have pressed Ctrl+XXX ... This does not work in markdown, instead etc." It also should not be an error, perhaps, but a warning or just info.

[williamstein]

Context: This happens when you hit shift+enter. Before we added this, we would regularly get support requests from people trying to use normal documents as if they are Jupyter notebooks are sage worksheets. After adding this, we never got any more such support requests.