cmd/dlv: require arg for trace subcommand (go-delve#2848)

Fixes go-delve#2845

cmd/dlv/cmds/commands.go

@@ -570,10 +570,14 @@ func traceCmd(cmd *cobra.Command, args []string) {
 
 		dlvArgs, targetArgs := splitArgs(cmd, args)
 		var dlvArgsLen = len(dlvArgs)
-		if dlvArgsLen == 1 {
+		switch dlvArgsLen {
+		case 0:
+			fmt.Fprintf(os.Stderr, "you must supply a regexp for functions to trace\n")
+			return 1
+		case 1:
 			regexp = args[0]
 			dlvArgs = dlvArgs[0:0]
-		} else if dlvArgsLen >= 2 {
+		default:
 			regexp = dlvArgs[dlvArgsLen-1]
 			dlvArgs = dlvArgs[:dlvArgsLen-1]
 		}
@@ -632,8 +636,7 @@ func traceCmd(cmd *cobra.Command, args []string) {
 			if traceUseEBPF {
 				err := client.CreateEBPFTracepoint(funcs[i])
 				if err != nil {
-					fmt.Fprintln(os.Stderr, err)
-					return 1
+					fmt.Fprintf(os.Stderr, "unable to set tracepoint on function %s: %#v\n", funcs[i], err)
 				}
 			} else {
 				// Fall back to breakpoint based tracing if we get an error.
@@ -645,13 +648,11 @@ func traceCmd(cmd *cobra.Command, args []string) {
 					LoadArgs:     &terminal.ShortLoadConfig,
 				})
 				if err != nil && !isBreakpointExistsErr(err) {
-					fmt.Fprintln(os.Stderr, err)
-					return 1
+					fmt.Fprintf(os.Stderr, "unable to set tracepoint on function %s: %#v\n", funcs[i], err)
 				}
 				addrs, err := client.FunctionReturnLocations(funcs[i])
 				if err != nil {
-					fmt.Fprintln(os.Stderr, err)
-					return 1
+					fmt.Fprintf(os.Stderr, "unable to set tracepoint on function %s: %#v\n", funcs[i], err)
 				}
 				for i := range addrs {
 					_, err = client.CreateBreakpoint(&api.Breakpoint{
@@ -662,8 +663,7 @@ func traceCmd(cmd *cobra.Command, args []string) {
 						LoadArgs:    &terminal.ShortLoadConfig,
 					})
 					if err != nil && !isBreakpointExistsErr(err) {
-						fmt.Fprintln(os.Stderr, err)
-						return 1
+						fmt.Fprintf(os.Stderr, "unable to set tracepoint on function %s: %#v\n", funcs[i], err)
 					}
 				}
 			}