resource server: improve IP attachment usability

[nicolai86]

this changeset ensures that if the IP attachment fails due to an invalid input, the creation server resource fails.
previously this was silently ignored, which led to #1.

make testacc TESTARGS="-run='TestAccScalewayServer'"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run='TestAccScalewayServer' -timeout 120m
?       github.com/terraform-providers/terraform-provider-scaleway    [no test files]
=== RUN   TestAccScalewayServer_importBasic
--- PASS: TestAccScalewayServer_importBasic (101.07s)
=== RUN   TestAccScalewayServer_Basic
--- PASS: TestAccScalewayServer_Basic (118.88s)
=== RUN   TestAccScalewayServer_ExistingIP
--- PASS: TestAccScalewayServer_ExistingIP (102.68s)
=== RUN   TestAccScalewayServer_Volumes
--- PASS: TestAccScalewayServer_Volumes (136.93s)
=== RUN   TestAccScalewayServer_SecurityGroup
--- PASS: TestAccScalewayServer_SecurityGroup (117.27s)
PASS
ok      github.com/terraform-providers/terraform-provider-scaleway/scaleway    576.847s

[grubernaut]

LGTM, thanks!

[nicolai86]

🎉thank you @grubernaut