Abort when opening the RDB file results in an error other than ENOENT.

This fixes cases where the RDB file does exist but can't be accessed for
any reason. For instance, when the Redis process doesn't have enough
permissions on the file.
Merge pull request #1001 from djanowski/fatal-errors-rdb-load

Abort when opening the RDB file results in an error other than ENOENT.