Issue machine-drivers#21: base: Return error when BundleName is not set

An empty bundle name does not make sense, so if its value is "", this
means it was not set while it's mandatory. Return an error when this
happens.

This fixes crc-org#21

libmachine/drivers/base.go

@@ -82,6 +82,9 @@ func (d *BaseDriver) ResolveStorePath(file string) string {
 }
 
 // Returns the name of the bundle which was used to create this machine
-func (d* BaseDriver) GetBundleName() (string, error) {
+func (d *BaseDriver) GetBundleName() (string, error) {
+	if d.BundleName == "" {
+		return "", errors.New("Bundle name is not set")
+	}
 	return d.BundleName, nil
 }