Raise pipelined meta protocol errors after draining

[mrattle]

Summary

• detect CLIENT_ERROR, SERVER_ERROR, and ERROR in pipelined meta responses
• keep draining pipelined responses through MN before raising the first protocol error
• cover first, middle, and last pipelined error positions in tests

Testing

• ruby -c lib/dalli/protocol/meta/response_processor.rb
• ruby -c lib/dalli/protocol/response_buffer.rb
• ruby -c lib/dalli/protocol/base.rb
• ruby -c lib/dalli/protocol/meta.rb
• ruby -c test/protocol/meta/test_response_processor.rb

Note: running the targeted test file with bundle exec currently fails locally due a Ruby/stringio binary incompatibility in this environment.