rewritemeta - don't delete the file if ruamel-yaml is not installed

While I setup a new VM, I might not yet have all the needed deps (The docs have an ... at apt-get, go figure) so when you run fdroid rewritemeta <appid> you'll get

DEBUG: Reading 'config.py'
INFO: Rewriting '<appid>'
Traceback (most recent call last):
  File "/home/strech/fdroidserver-0408/fdroidserver/metadata.py", line 1120, in write_yaml
    import ruamel.yaml
ImportError: No module named 'ruamel'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/local/bin/fdroid", line 170, in <module>
    main()
  File "/usr/local/bin/fdroid", line 146, in main
    mod.main()
  File "/home/strech/fdroidserver-0408/fdroidserver/rewritemeta.py", line 111, in main
    metadata.write_metadata(base + '.' + to_ext, app)
  File "/home/strech/fdroidserver-0408/fdroidserver/metadata.py", line 1595, in write_metadata
    raise e
  File "/home/strech/fdroidserver-0408/fdroidserver/metadata.py", line 1592, in write_metadata
    return write_yaml(mf, app)
  File "/home/strech/fdroidserver-0408/fdroidserver/metadata.py", line 1122, in write_yaml
    raise FDroidException('ruamel.yaml not instlled, can not write metadata.') from e
fdroidserver.exception.FDroidException: ruamel.yaml not instlled, can not write metadata.

You install it, then run again, only to discover that the metadata file for your app is gone.