This commit fixes serving of newly added files

This commit handles the issue in serve command which cannot host
new files. It is worth noting that new file content won't appear
until the tags are added.

Fixed #19
Actually I don't think we should separate `FileNotFoundError` and `Exception` handlers from each other. I propose to handle only `Exception`. I believe that `FileNotFoundError` is incredible rare for serve command, so we can skip it.