filter: Propagate schema validation error to end user

cloudflare#848 introduced better validation for filters using the Terraform
schema. Despite handling the error, the error wasn't returned to the end
user and instead the RPC exception is shown the end user.

```
Error: rpc error: code = Unavailable desc = transport is closing
```

This fixes the user experience by returning the errors which now passes
the exception onto the end user instead of swallowing it or causing a
panic.

cloudflare/resource_cloudflare_filter.go

@@ -10,6 +10,7 @@ import (
 	cloudflare "github.com/cloudflare/cloudflare-go"
 	"github.com/hashicorp/terraform-plugin-sdk/helper/schema"
 	"github.com/hashicorp/terraform-plugin-sdk/helper/validation"
+	"github.com/pkg/errors"
 )
 
 func resourceCloudflareFilter() *schema.Resource {
@@ -45,7 +46,8 @@ func resourceCloudflareFilter() *schema.Resource {
 					// interface available that holds the configured client.
 					api, err := cloudflare.New(os.Getenv("CLOUDFLARE_API_KEY"), os.Getenv("CLOUDFLARE_EMAIL"))
 					if err != nil {
-						log.Fatal(err)
+						errs = append(errs, errors.New("cloudflare_api_key and cloudflare_email are required for validating filter expressions but they are missing"))
+						return
 					}
 
 					expression := val.(string)

cloudflare/resource_cloudflare_filter_test.go

@@ -82,6 +82,44 @@ func TestAccFilterInvalid(t *testing.T) {
 		},
 	})
 }
+
+func TestAccFilterMissingCredentials(t *testing.T) {
+	// Intentionally unset all credentials to trigger the lack of credentials
+	// check schema validation.
+	if os.Getenv("CLOUDFLARE_API_TOKEN") != "" {
+		defer func(apiToken string) {
+			os.Setenv("CLOUDFLARE_API_TOKEN", apiToken)
+		}(os.Getenv("CLOUDFLARE_API_TOKEN"))
+		os.Setenv("CLOUDFLARE_API_TOKEN", "")
+	}
+
+	if os.Getenv("CLOUDFLARE_API_KEY") != "" {
+		defer func(apiKey string) {
+			os.Setenv("CLOUDFLARE_API_KEY", apiKey)
+		}(os.Getenv("CLOUDFLARE_API_KEY"))
+		os.Setenv("CLOUDFLARE_API_KEY", "")
+	}
+
+	if os.Getenv("CLOUDFLARE_EMAIL") != "" {
+		defer func(email string) {
+			os.Setenv("CLOUDFLARE_EMAIL", email)
+		}(os.Getenv("CLOUDFLARE_EMAIL"))
+		os.Setenv("CLOUDFLARE_EMAIL", "")
+	}
+
+	rnd := generateRandomResourceName()
+	zoneID := os.Getenv("CLOUDFLARE_ZONE_ID")
+
+	resource.Test(t, resource.TestCase{
+		Providers: testAccProviders,
+		Steps: []resource.TestStep{
+			{
+				Config:      testFilterConfig(rnd, zoneID, "true", "this is notes", "invalid expression"),
+				ExpectError: regexp.MustCompile("cloudflare_api_key and cloudflare_email are required for validating filter expressions but they are missing"),
+			},
+		},
+	})
+}
 func TestAccFilterInvalidOver4kbString(t *testing.T) {
 	rnd := generateRandomResourceName()
 	zoneID := os.Getenv("CLOUDFLARE_ZONE_ID")