Reference Panel: preview pane fails to load blob if syntax highlighting times out

Created by: olafurpg

To reproduce, visit https://sourcegraph.com/github.com/apache/spark/-/blob/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala?L58:7&popover=pinned#tab=references and open the preview for a result in AstBuilder.scala

To fix this issue, we should display the unhighlighted code in the preview pane, similar to how the main blob view displays unhighlighted code.