curl_xml plugin: Check for a curl_easy_perform() error first

The value of CURLINFO_RESPONSE_CODE isn't valid otherwise.
Also, use the symbolic name CURLE_OK in all plugins where
appropriate.

Change-Id: I17ae9c7eea393ee4641296b5484c93809a662dd9
Signed-off-by: Florian Forster <octo@collectd.org>
src/filter_chain.c: Fixed typos in error messages

Signed-off-by: Florian Forster <octo@collectd.org>