Fix a pair of bugs with crud.Insert --

 * If no pkey is provided, omit the RETURNING. This is used for tables
   that don't have a generated pkey.
 * If the INSERT query returns no rows, pass back `rows.Err()` which
   contains the error, if any, during query execution.

I also accidentially ran `go fmt` which blew up the diff. Sorry :(
Merge pull request #1 from lcampbell/refer-bug-fixes

Fix a pair of bugs with crud.Insert --