docs: Add missing changelog entry and comment (#2451)

Follow-up on https://github.com/tendermint/tendermint/pull/2411

CHANGELOG_PENDING.md

@@ -26,6 +26,7 @@ FEATURES:
   * \#2310 Mempool is now aware of the MaxGas requirement
 
 IMPROVEMENTS:
+- [libs/db] \#2371 Output error instead of panic when the given db_backend is not initialised (@bradyjoestar)
 - [mempool] [\#2399](https://github.com/tendermint/tendermint/issues/2399) Make mempool cache a proper LRU (@bradyjoestar)
 - [types] add Address to GenesisValidator [\#1714](https://github.com/tendermint/tendermint/issues/1714)
 - [metrics] `consensus.block_interval_metrics` is now gauge, not histogram (you will be able to see spikes, if any)

libs/db/db.go

@@ -30,19 +30,23 @@ func registerDBCreator(backend DBBackendType, creator dbCreator, force bool) {
 	backends[backend] = creator
 }
 
+// NewDB creates a new database of type backend with the given name.
+// NOTE: function panics if:
+//   - backend is unknown (not registered)
+//   - creator function, provided during registration, returns error
 func NewDB(name string, backend DBBackendType, dir string) DB {
 	dbCreator, ok := backends[backend]
-
 	if !ok {
-		var keys []string
-		for k, _ := range backends {
-			keys = append(keys, string(k))
+		keys := make([]string, len(backends))
+		i := 0
+		for k := range backends {
+			keys[i] = string(k)
+			i++
 		}
 		panic(fmt.Sprintf("Unknown db_backend %s, expected either %s", backend, strings.Join(keys, " or ")))
 	}
 
 	db, err := dbCreator(name, dir)
-
 	if err != nil {
 		panic(fmt.Sprintf("Error initializing DB: %v", err))
 	}