[Serializer] Fixed the error handling when decoding invalid XML to avoid a Warning

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Without the exception being thrown here, the call to ``simplexml_import_dom`` throws a warning saying ``Invalid Nodetype to import``.
This has been reported by @Tobion after we refactored FOSRestBundle to use this XmlEncoder instead of a duplication of a previous version of the class: https://github.com/FriendsOfSymfony/FOSRestBundle/pull/583/files#r7221045