Handle main branch build errors

[tobie]

In order to build an HTML diff, PR Preview must be able to successfully build the main branch.

If the main branch can't be built, that throws an error, regardless of whether or not the branch build itself can be fixed.

This is confusing to users and must be corrected.

Plan:

If building the branch itself doesn't fail, build and display that. Add an error message explaining why the diff cannot be shown, and/or attempt at forcing a build of the main branch.