spool to disk: more descriptive error message (#7338)

When the file backing the queue.spool cannot be open or created, beats
fail with a rather vague error message:

  Exiting: error initializing publisher: invalid argument

This happens when the file resides in a filesystem that doesn't support
mmap, like a shared virtualbox directory.

Now it will output something a little bit more descriptive:

Exiting: error initializing publisher: spool queue: failed to open file
at path '/vagrant/src/github.com/elastic/beats/auditbeat/data/spool.dat':
invalid argument

(cherry picked from commit 0bfb541)

libbeat/publisher/queue/spool/spool.go

@@ -6,6 +6,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/pkg/errors"
+
 	"github.com/elastic/beats/libbeat/common/atomic"
 	"github.com/elastic/beats/libbeat/publisher/queue"
 	"github.com/elastic/go-txfile"
@@ -86,7 +88,7 @@ func NewSpool(logger logger, path string, settings Settings) (*Spool, error) {
 
 	f, err := txfile.Open(path, mode, settings.File)
 	if err != nil {
-		return nil, err
+		return nil, errors.Wrapf(err, "spool queue: failed to open file at path '%s'", path)
 	}
 	defer ifNotOK(&ok, ignoreErr(f.Close))