Improve scan error messages
Improve scan error messages

- Use Redis names in messages instead of Go type names. For example, use
  "bulk string" instead of "[]uint8".
- To aid debugging, add field names and slice indexes to error messages
  as appropriate.
- Prefix errors with the function name.

Fixes #142

mbrukman/redigo,beni55/redigo,nativetouch/redigo,kylebrandt/redigo,gomodule/redigo,garyburd/redigo,linkeo/redigo,lvtiendev/redigo,brodock/redigo,kpumuk/redigo,outrightmental/go-redigo,stvp/redigo