Error handling should be reviewed in resourceXXXRead() functions

[maksym-nazarenko]

This issue was revealed while debugging #63

When some resources are read by Terraform, in case of error the id is set to "" (empty string) which is okay in case of Resource not found - it means that Terraform should remove it from its state.
But in case of another kind of error, e.g. Invalid credentials or Host not found, it should not be the case.

The DnsRecord handles it properly:

if _, ok := err.(*client.NotFound); ok {
	d.SetId("")
	return nil
}

but, for example, Script and Pool don't:

if err != nil {
	d.SetId("")
	return nil
}

So, in case of wrong credentials, Terraform is trying to delete resources.

It, probably, will be mitigated with #64 , but it is far from even Ready for review state.

[ddelnano]

The fix will be included in v0.9.1.