Improve the error handling when starting a session

If the command cannot be started, or the proxy server can't be reached,
exit early and a bit nicer.

main.go

@@ -125,6 +125,13 @@ Flags:
 		os.Exit(1)
 	}
 
+	ptyMaster := ptyMasterNew()
+	err := ptyMaster.Start(*commandName, strings.Fields(*commandArgs))
+	if err != nil {
+		log.Errorf("Cannot start the %s command: %s", *commandName, err.Error())
+		return
+	}
+
 	sessionID := "local"
 	if *publicSession {
 		proxy, err := proxy.NewProxyConnection(*listenAddress, *proxyServerAddress, *noTLS)
@@ -145,11 +152,8 @@ Flags:
 	fmt.Printf("Press Enter to continue!\n")
 	bufio.NewReader(os.Stdin).ReadString('\n')
 
-	ptyMaster := ptyMasterNew()
+	ptyMaster.MakeRaw()
 	defer ptyMaster.Restore()
-
-	ptyMaster.Start(*commandName, strings.Fields(*commandArgs))
-
 	var pty server.PTYHandler = ptyMaster
 	if *readOnly {
 		pty = &nilPTY{}

pty_master.go

@@ -31,15 +31,6 @@ func isStdinTerminal() bool {
 }
 
 func (pty *ptyMaster) Start(command string, args []string) (err error) {
-	// Save the initial state of the terminal, before making it RAW. Note that this terminal is the
-	// terminal under which the tty-share command has been started, and it's identified via the
-	// stdin file descriptor (0 in this case)
-	// We need to make this terminal RAW so that when the command (passed here as a string, a shell
-	// usually), is receiving all the input, including the special characters:
-	// so no SIGINT for Ctrl-C, but the RAW character data, so no line discipline.
-	// Read more here: https://www.linusakesson.net/programming/tty/
-	pty.terminalInitState, err = terminal.MakeRaw(0)
-
 	pty.command = exec.Command(command, args...)
 	pty.ptyFile, err = ptyDevice.Start(pty.command)
 
@@ -53,6 +44,19 @@ func (pty *ptyMaster) Start(command string, args []string) (err error) {
 	return
 }
 
+func (pty *ptyMaster) MakeRaw() (err error) {
+
+	// Save the initial state of the terminal, before making it RAW. Note that this terminal is the
+	// terminal under which the tty-share command has been started, and it's identified via the
+	// stdin file descriptor (0 in this case)
+	// We need to make this terminal RAW so that when the command (passed here as a string, a shell
+	// usually), is receiving all the input, including the special characters:
+	// so no SIGINT for Ctrl-C, but the RAW character data, so no line discipline.
+	// Read more here: https://www.linusakesson.net/programming/tty/
+	pty.terminalInitState, err = terminal.MakeRaw(int(os.Stdin.Fd()))
+	return
+}
+
 func (pty *ptyMaster) SetWinChangeCB(winChangedCB onWindowChangedCB) {
 	// Start listening for window changes
 	go onWindowChanges(func(cols, rows int) {