Default fork choice value and intermediates for block import parameters (#4652)

* consensus, pow: intermediate separation and fail

* Fix compiles

* Update primitives/consensus/common/src/block_import.rs

Co-Authored-By: Robert Habermeier <rphmeier@gmail.com>

* Update primitives/consensus/common/src/block_import.rs

Co-Authored-By: Robert Habermeier <rphmeier@gmail.com>

* Document what None means for `fork_choice` in block import params

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

substrate/primitives/blockchain/src/error.rs

@@ -126,6 +126,9 @@ pub enum Error {
 	/// Invalid calculated state root on block import.
 	#[display(fmt = "Calculated state root does not match.")]
 	InvalidStateRoot,
+	/// Incomplete block import pipeline.
+	#[display(fmt = "Incomplete block import pipeline.")]
+	IncompletePipeline,
 	/// A convenience variant for String
 	#[display(fmt = "{}", _0)]
 	Msg(String),