catch and report start cmd failure rather than panicing (#676)

* do not report the process as running if it failed to start

* change the return values on error to match what the other platforms produce

runner/engine.go

@@ -538,7 +538,13 @@ func (e *Engine) runBin() error {
 			default:
 				e.procKillWg.Add(1)
 				command := strings.Join(append([]string{e.config.Build.Bin}, e.runArgs...), " ")
-				cmd, stdout, stderr, _ := e.startCmd(command)
+				cmd, stdout, stderr, err := e.startCmd(command)
+				if err != nil {
+					e.mainLog("failed to start %s, error: %s", e.config.rel(e.config.binPath()), err.Error())
+					close(killCh)
+					continue
+				}
+
 				processExit := make(chan struct{})
 				e.mainDebug("running process pid %v", cmd.Process.Pid)
 				if e.config.Proxy.Enabled {

runner/util_linux.go

@@ -31,5 +31,8 @@ func (e *Engine) killCmd(cmd *exec.Cmd) (pid int, err error) {
 func (e *Engine) startCmd(cmd string) (*exec.Cmd, io.ReadCloser, io.ReadCloser, error) {
 	c := exec.Command("/bin/sh", "-c", cmd)
 	f, err := pty.Start(c)
-	return c, f, f, err
+	if err != nil {
+		return nil, nil, nil, err
+	}
+	return c, f, f, nil
 }