Ensure precompile check works even w/o extension.

Fixes a problem where this correctly raises an error:

```
<%= stylesheet_link_tag("admin.css") %>
```

But this does not:

```
<%= stylesheet_link_tag("admin") %>
```

This is because the Rails tag helpers perform some magic to append the appropriate extension when searching for a matching asset. We have to do a similar trick. Otherwise a extension-less source will be matched by the
Sprockets `LOOSE_APP_ASSETS` proc and no precompile error will be raised.