Remove some unwrap/expect in chain crate (#2621)

* Return Result instead of calling expect in root(). It would kill peer's thread. Perhaps we should ban this peer as malicious.
* Remove some unwraps

chain/src/txhashset/rewindable_kernel_view.rs

@@ -68,7 +68,8 @@ impl<'a> RewindableKernelView<'a> {
 	/// fast sync where a reorg past the horizon could allow a whole rewrite of
 	/// the kernel set.
 	pub fn validate_root(&self) -> Result<(), Error> {
-		if self.pmmr.root() != self.header.kernel_root {
+		let root = self.pmmr.root().map_err(|_| ErrorKind::InvalidRoot)?;
+		if root != self.header.kernel_root {
 			return Err(ErrorKind::InvalidTxHashSet(format!(
 				"Kernel root at {} does not match",
 				self.header.height

chain/src/txhashset/txhashset.rs

@@ -272,9 +272,11 @@ impl TxHashSet {
 	}
 
 	/// build a new merkle proof for the given position.
-	pub fn merkle_proof(&mut self, commit: Commitment) -> Result<MerkleProof, String> {
-		let pos = self.commit_index.get_output_pos(&commit).unwrap();
-		PMMR::at(&mut self.output_pmmr_h.backend, self.output_pmmr_h.last_pos).merkle_proof(pos)
+	pub fn merkle_proof(&mut self, commit: Commitment) -> Result<MerkleProof, Error> {
+		let pos = self.commit_index.get_output_pos(&commit)?;
+		PMMR::at(&mut self.output_pmmr_h.backend, self.output_pmmr_h.last_pos)
+			.merkle_proof(pos)
+			.map_err(|_| ErrorKind::MerkleProof.into())
 	}
 
 	/// Compact the MMR data files and flush the rm logs
@@ -1590,6 +1592,5 @@ pub fn input_pos_to_rewind(
 		current = batch.get_previous_header(&current)?;
 	}
 
-	let bitmap = bitmap_fast_or(None, &mut block_input_bitmaps).unwrap();
-	Ok(bitmap)
+	bitmap_fast_or(None, &mut block_input_bitmaps).ok_or_else(|| ErrorKind::Bitmap.into())
 }