cmd/geth: user friendly light miner error

cmd/geth/main.go

@@ -278,9 +278,12 @@ func startNode(ctx *cli.Context, stack *node.Node) {
 	// Start auxiliary services if enabled
 	if ctx.GlobalBool(utils.MiningEnabledFlag.Name) || ctx.GlobalBool(utils.DeveloperFlag.Name) {
 		// Mining only makes sense if a full Ethereum node is running
+		if ctx.GlobalBool(utils.LightModeFlag.Name) || ctx.GlobalString(utils.SyncModeFlag.Name) == "light" {
+			utils.Fatalf("Light clients do not support mining")
+		}
 		var ethereum *eth.Ethereum
 		if err := stack.Service(&ethereum); err != nil {
-			utils.Fatalf("ethereum service not running: %v", err)
+			utils.Fatalf("Ethereum service not running: %v", err)
 		}
 		// Use a reduced number of threads if requested
 		if threads := ctx.GlobalInt(utils.MinerThreadsFlag.Name); threads > 0 {