- [ABI BREAK] add an ErrorType option to CacheSetHelper

* cmdline/apt-cache.cc:
  - use Notice instead of Error in the CacheSetHelper messages
    for compat reasons. Otherwise tools like sbuild blow up
* debian/control:
  - remove libcurl3-gnutls-dev alternative as the package is gone
  - increase needed version of libcurl4-gnutls-dev to >= 7.19.0
    as we use CURLOPT_{ISSUERCERT,CRLFILE} (Closes: #589642)