Give a more informative error when using redshift

When an error occurs on redshift, it often looks like this

```
InternalError: (psycopg2.InternalError) Load into table 'tmp0' failed. Check 'stl_load_errors' system table for details.
```

This is wholly uninformative. Often times the error is something like an incorrect delimiter or failure to specify that a file was compressed. It'd be nice if we could query this table and report a more informative message to the user.