Fix feed importing when URI's cannot be parsed when expanding urls

Again relates to #261.

I noticed http://aphyr.com/posts.atom could not be imported because it contains a pretty weird looking url [0] (which Chrome does handle fine btw). The importing process would just blow up because a `URI::InvalidURIError` would be raised in the `expand_asbolute_urls` method.

I've rescued that specific error in the `expand_absolute_urls` method since it's an expected exception. If an url cannot be parsed I think it's best to just leave it as is.

In general I think the feed importing process could use some improvements in not blowing up when a single article fails to import. Would be cool to continue importing even if a single article fails to parse. We could then use a Feed status to indicate which posts failed parsing. What do you think @swanson?

[0] https://github.com/aphyr/jepsen/blob/1403f2d6e61c595bafede0d404fd4a893371c036/elasticsearch/src/jepsen/system/elasticsearch.clj#L161-L226.%20Then%20we'll%20write%20a%20%5Bregister%20test%5D(https://github.com/aphyr/jepsen/blob/1403f2d6e61c595bafede0d404fd4a893371c036/elasticsearch/test/jepsen/system/elasticsearch_test.clj#L18-L50
Good catch, fixed.