Convert all HTTP client errors to SolrError
Convert all HTTP client errors to SolrError

This commit ensures that an outside caller can handle all HTTP-related errors by catching SolrError without knowing whether the exception class is owned by requests, urllib3, or httplib.
django-haystack/pysolr,shasha79/pysolr,rokaka/pysolr,upayavira/pysolr,CANTUS-Project/pysolr-tornado,toastdriven/pysolr,CANTUS-Project/pysolr-tornado,mylanium/pysolr,django-searchstack/skisolr,swistakm/pysolr,mylanium/pysolr,upayavira/pysolr,rokaka/pysolr,django-searchstack/skisolr,mbeacom/pysolr,toastdriven/pysolr,shasha79/pysolr,swistakm/pysolr,mbeacom/pysolr,django-haystack/pysolr