Throw exception when metaCPAN search response is error

[steverichey]

This line returns null when the package search fails. However, this causes other unexpected errors, such as ArgumentNullException when called by Latest(...), which is a public-facing method.

This is mainly an issue when using a newer version of Elasticsearch.Net, which is unlikely to happen on accident, but it surfaces a need for a better user-facing error in this case.