k8s: Handle error from GetCredentials and protect against panic.

[andrewsomething]

For some reason, we only return an error when attempting to fetch a cluster's credentials if it is a 404 response. This can lead to a panic in other error cases as we attempt to flatten the credential which are nil. This handles the error properly and protect against a panic in flattenCredentials.

Example panic:

Stack trace from the terraform-provider-digitalocean_v2.30.0 plugin:
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xcfad4c]
goroutine 465 [running]:
github.com/digitalocean/terraform-provider-digitalocean/digitalocean/kubernetes.flattenCredentials({0xc000b61490, 0xc}, {0xc000b6147c, 0x4}, 0x0)
github.com/digitalocean/terraform-provider-digitalocean/digitalocean/kubernetes/resource_kubernetes_cluster.go:680 +0x4c
github.com/digitalocean/terraform-provider-digitalocean/digitalocean/kubernetes.digitaloceanKubernetesClusterRead(0xc00012a580, 0xc000736480, 0x1974200?)
github.com/digitalocean/terraform-provider-digitalocean/digitalocean/kubernetes/resource_kubernetes_cluster.go:409 +0xccc
github.com/digitalocean/terraform-provider-digitalocean/digitalocean/kubernetes.resourceDigitalOceanKubernetesClusterRead({0x12938c0?, 0xc000cffaa0?}, 0xc00072e480, {0xe80840?, 0xc000671890?})
github.com/digitalocean/terraform-provider-digitalocean/digitalocean/kubernetes/resource_kubernetes_cluster.go:337 +0x1c9
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).read(0xc00056ce00, {0x12938c0, 0xc000cffaa0}, 0xd?, {0xe80840, 0xc000671890})
github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:724 +0x11b
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).RefreshWithoutUpgrade(0xc00056ce00, {0x12938c0, 0xc000cffaa0}, 0xc0007dc8f0, {0xe80840, 0xc000671890})
github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:1015 +0x549
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).ReadResource(0xc0004137d0, {0x12938c0?, 0xc000cff170?}, 0xc00064afc0)
github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/grpc_provider.go:613 +0x48a
github.com/hashicorp/terraform-plugin-go/tfprotov5/tf5server.(*server).ReadResource(0xc000092140, {0x12938c0?, 0xc000cfe1e0?}, 0xc000764120)
github.com/hashicorp/[email protected]/tfprotov5/tf5server/server.go:748 +0x48a
github.com/hashicorp/terraform-plugin-go/tfprotov5/internal/tfplugin5._Provider_ReadResource_Handler({0xf61920?, 0xc000092140}, {0x12938c0, 0xc000cfe1e0}, 0xc0007def50, 0x0)
github.com/hashicorp/[email protected]/tfprotov5/internal/tfplugin5/tfplugin5_grpc.pb.go:349 +0x169
google.golang.org/grpc.(*Server).processUnaryRPC(0xc00041c000, {0x12984e0, 0xc0000b2680}, 0xc000a05560, 0xc000585620, 0x1937430, 0x0)
google.golang.org/[email protected]/server.go:1336 +0xd67
google.golang.org/grpc.(*Server).handleStream(0xc00041c000, {0x12984e0, 0xc0000b2680}, 0xc000a05560, 0x0)
google.golang.org/[email protected]/server.go:1704 +0x9e7
google.golang.org/grpc.(*Server).serveStreams.func1.2()
google.golang.org/[email protected]/server.go:965 +0x8d
created by google.golang.org/grpc.(*Server).serveStreams.func1 in goroutine 13
google.golang.org/[email protected]/server.go:963 +0x246