Log an upload error closer to when it occurs

When there is an error returned from the Upload it is propagated back
to the main loop and may have additional log messages before the actual
error is displayed. This displays a log message close to where the error
actually occurred.

provisioner/file/provisioner.go

@@ -79,5 +79,9 @@ func (p *Provisioner) Provision(ui packer.Ui, comm packer.Communicator) error {
 	}
 	defer f.Close()
 
-	return comm.Upload(p.config.Destination, f)
+	err = comm.Upload(p.config.Destination, f)
+	if err != nil {
+		ui.Error(fmt.Sprintf("Upload failed: %s", err))
+	}
+	return err
 }