This includes removing the error return value from Permalink and RelPermalink.
We ignore that error all over the place, so we might as well remove it.
Updates #2297
To make it easier to follow and understand.
Both building and rebuilding now follow a four step flow:
1. Init
2. Process
3. Assemble
4. Render
And now there are only one Build method, used for both builds and rebuilds.
Updates #2297
