"better" error handling

@mswain is this crazy ? really don't like reaching into .transport ivars but errors will come out looking like this:
```
=> #<ThriftClient::TransportError: Error on server 127.0.0.1:9160 : multiget_slice column name must not be empty>
```
instead of:
```
=> #<ThriftClient::TransportError: No live servers in [#<ThriftHelpers::Server:0x007fe1f155f9d0 @connection_string="127.0.0.1:9160", @client_class=CassandraThrift::Cassandra::Client, @options={:protocol=>Thrift::BinaryProtocolAccelerated, :protocol_extra_params=>[], :transport=>Thrift::Socket, :transport_wrapper=>Thrift::FramedTransport, :raise=>true, :defaults=>{}, :exception_classes=>[IOError, Thrift::Exception, Thrift::ApplicationException, Thrift::TransportException], :exception_class_overrides=>[], :retries=>3, :server_retry_period=>1, :server_max_requests=>nil, :retry_overrides=>{}, :wrapped_exception_classes=>[Thrift::ApplicationException, Thrift::TransportException], :connect_timeout=>1, :timeout=>10, :timeout_overrides=>{}, :cached_connections=>false, :thrift_client_class=>ThriftClient}, @cached=false, @marked_down_til=2013-11-16 17:24:43 +0000, @connection=#<ThriftHelpers::Connection::Socket:0x007fe1f1a632b0 @transport=#<Thrift::FramedTransport:0x007fe1f1a62ec8 @transport=#<Thrift::Socket:0x007fe1f1a62fe0 @host="127.0.0.1", @port=9160, @timeout=10, @desc="127.0.0.1:9160", @handle=nil>, @rbuf="\x80\x01\x00\x02\x00\x00\x00\x0Emultiget_slice\x00\x00\x00\x00\f\x00\x01\v\x00\x01\x00\x00\x00\x1Dcolumn name must not be empty\x00\x00", @wbuf="", @read=true, @write=true, @index=67>, @transport_wrapper=Thrift::FramedTransport, @server="127.0.0.1:9160", @timeout=1>, @client=nil>].>
```