Don't return Permerror on no host or empty TXT record

[trtstm]

Fix for #3. But don't know if it's a good way to solve it...
The problem is that net.LookupTXT does not return a specific error if there was no TXT record. It just returns "no such host".

[maarten-postmastery]

Why not just return the underlying error?

record, err := spf.dns.GetSPFRecord(domain)
if err != nil {
    return nil, err
}

When err is nil, you should expect a valid SPF record. Policy about what to do when there is an SPF lookup error could be implemented on a higher level.

In any case, testing for error strings is bad practice. See this post and this post about inspecting errors.

[trtstm]

Ye you are right, thats what I thought when reading #3. I just created this fix without considering whether this is something that should be fixed :). That's why I assigned @DenBeke to validate it.
So this pull request can be deleted if @DenBeke agrees with it.