Rescue tilt load error

Here's the remaining piece of this fix. Your changes were focused on rendering errors. These changes are focused on load errors that can occur when the Tilt resolution is performed. The behavior helps the user to understand that a content file is being copied unprocessed because a required gem is missing.

Here's what the user will see if a Markdown file is present but no Markdown libraries are available:

```
Using profile: development
Generating site: http://localhost:4242
Copying /path/to/site/README.md to generated site without processing; missing required gem -- redcarpet (or equivalent)
```

Notice that I used the phrase "missing required gem" (replacing the default message about a load error). I also added "or equivalent" so that they know that the gem (in this case redcarpet) isn't their only option.