lint fails on tool dependency packages of just tool_dependencies.xml

e.g. using https://github.com/peterjc/galaxy_blast as an example,

``` bash
$ planemo --version
planemo, version 0.18.1.dev0
$ ls -a packages/package_blast_plus_2_2_31
. .. .shed.yml tool_dependencies.xml
$ planemo lint --report_level all --fail_level error packages/package_blast_plus_2_2_31 ; echo "Return value $?"
Return value 2
```

The same happens if recursing:

```
$ planemo lint --report_level all --fail_level error -r packages/ ; echo "Return value $?"
Return value 2
```

I expected `planemo lint` to at least check `tool_dependencies.xml` was a valid XML file, and return 0 (success), even if nothing more rigorous was done.