only catch actual ImportError, which is self-explaining, too

Not being able to import a module raises an ImportError, so that is the only one to be ignored,not any exception such as e.g. a typo in the imporrt statement or any errors in the markdown module.
As the comment does not convey any more informatin that "except ImportError" already does, I removed it. If you do prefer to have a comment there, I'd suggest something like "markdown is optional".