cs: exit if SwitchToConsensus fails (#3706)

Refs #3656

CHANGELOG_PENDING.md

@@ -32,6 +32,7 @@
 ### FEATURES:
 
 ### IMPROVEMENTS:
+- [consensus] \#3656 Exit if SwitchToConsensus fails
 - [p2p] \#3666 Add per channel telemetry to improve reactor observability
 - [rpc] [\#3686](https://github.com/tendermint/tendermint/pull/3686) `HTTPClient#Call` returns wrapped errors, so a caller could use `errors.Cause` to retrieve an error code. (@wooparadog)
 - [abci/examples] \#3659 Change validator update tx format (incl. expected pubkey format, which is base64 now) (@needkane)

consensus/reactor.go

@@ -116,8 +116,13 @@ func (conR *ConsensusReactor) SwitchToConsensus(state sm.State, blocksSynced int
 	}
 	err := conR.conS.Start()
 	if err != nil {
-		conR.Logger.Error("Error starting conS", "err", err)
-		return
+		panic(fmt.Sprintf(`Failed to start consensus state: %v
+
+conS:
+%+v
+
+conR:
+%+v`, err, conR.conS, conR))
 	}
 }