When we know to look into the index pages collection, do that:
```
benchmark old ns/op new ns/op delta
BenchmarkGetPage-4 51483 7072 -86.26%
benchmark old allocs new allocs delta
BenchmarkGetPage-4 71 71 +0.00%
benchmark old bytes new bytes delta
BenchmarkGetPage-4 2648 2648 +0.00%
```
This commit also returns an error if .Site.GetPage is called with the regular Page Kind, as that is currently not supported.
Fixes#3503
This commit fixes two different, but related issues:
1) Live-reload when a new shortcode was defined in the content file before the shortcode itself was created.
2) Live-reload when a newly defined shortcode changed its "inner content" status.
This commit also improves the shortcode related error messages to include the full path to the content file in question.
Fixes#3156
