do not pass nil to genericError

currently genericError constructors require not-nil error to be able to read
its Error() message

Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com>

container_linux.go

@@ -206,7 +206,7 @@ func (c *linuxContainer) Destroy() error {
 		return err
 	}
 	if status != Destroyed {
-		return newGenericError(nil, ContainerNotStopped)
+		return newGenericError(fmt.Errorf("container is not destroyed"), ContainerNotStopped)
 	}
 	if !c.config.Namespaces.Contains(configs.NEWPID) {
 		if err := killCgroupProcesses(c.cgroupManager); err != nil {

process.go

@@ -1,6 +1,7 @@
 package libcontainer
 
 import (
+	"fmt"
 	"io"
 	"os"
 )
@@ -46,23 +47,23 @@ type Process struct {
 // Wait releases any resources associated with the Process
 func (p Process) Wait() (*os.ProcessState, error) {
 	if p.ops == nil {
-		return nil, newGenericError(nil, ProcessNotExecuted)
+		return nil, newGenericError(fmt.Errorf("invalid process"), ProcessNotExecuted)
 	}
 	return p.ops.wait()
 }
 
 // Pid returns the process ID
 func (p Process) Pid() (int, error) {
 	if p.ops == nil {
-		return -1, newGenericError(nil, ProcessNotExecuted)
+		return -1, newGenericError(fmt.Errorf("invalid process"), ProcessNotExecuted)
 	}
 	return p.ops.pid(), nil
 }
 
 // Signal sends a signal to the Process.
 func (p Process) Signal(sig os.Signal) error {
 	if p.ops == nil {
-		return newGenericError(nil, ProcessNotExecuted)
+		return newGenericError(fmt.Errorf("invalid process"), ProcessNotExecuted)
 	}
 	return p.ops.signal(sig)
 }