Return buffered text from log endpoint if decoding fails

To see why I think this is a good change lets look at why I am making it

My disk was full, which means GC was happening agressively. So by the
time I called the logging endpoint from the SDK, the logs were GC'd

The error I was getting before was:
```
invalid character 'i' in literal false (expecting 'l')
```

Now the error I get is:
```
failed to decode log endpoint response as JSON: "failed to list entries: open /tmp/nomad.data.4219353875/alloc/f11fee50-2b66-a7a2-d3ec-8442cb3d557a/alloc/logs: no such file or directory"
```

Still not super descriptive but much more debugable

api/fs.go

@@ -4,10 +4,13 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net"
 	"strconv"
 	"sync"
 	"time"
+
+	"github.com/hashicorp/go-multierror"
 )
 
 const (
@@ -288,7 +291,12 @@ func (a *AllocFS) Logs(alloc *Allocation, follow bool, task, logType, origin str
 				if err == io.EOF || err == io.ErrClosedPipe {
 					close(frames)
 				} else {
-					errCh <- err
+					buf, err2 := ioutil.ReadAll(dec.Buffered())
+					if err2 != nil {
+						errCh <- fmt.Errorf("failed to decode and failed to read buffered data: %w", multierror.Append(err, err2))
+					} else {
+						errCh <- fmt.Errorf("failed to decode log endpoint response as JSON: %q", buf)
+					}
 				}
 				return
 			}