Cleanly handle missing stream error on COPY operation. Closes #241
Rework handling of missing stream object for copy ops

This version works better (doesn't throw) but also doesn't report
any error, which is not good
Send backend a CopyFail when no stream is defined to copy from
Merge pull request #242 from CartoDB/master-error-on-missing-stream

Cleanly handle missing stream error on COPY operation. Closes #241