Return error for multiple config options set

An error is now returned when a more than one storage configuration has
been set. Previously, this would not return an error and simply use what
-ever the user had set that was first in the storage switch options.

Closes #853

internal/storage/types/location_config.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"net/url"
+	"reflect"
 
 	"github.com/aws/aws-sdk-go/aws/credentials"
 	"github.com/aws/aws-sdk-go/aws/session"
@@ -53,14 +54,21 @@ func (c *LocationConfig) UnmarshalJSON(blob []byte) error {
 		return err
 	}
 
+	errMultipleConfig := errors.New("multiple config values have been assigned")
 	switch {
-	// TODO: return error if we have more than one config assigned (mutually exclusive)
 	case types.S3 != nil:
 		c.Value = types.S3
 	case types.SFTPConfig != nil:
+		if reflect.ValueOf(c.Value).IsValid() {
+			return errMultipleConfig
+		}
 		c.Value = types.SFTPConfig
 	case types.URLConfig != nil:
+		if reflect.ValueOf(c.Value).IsValid() {
+			return errMultipleConfig
+		}
 		c.Value = types.URLConfig
+
 	default:
 		return errors.New("undefined configuration document")
 	}
@@ -143,6 +151,5 @@ func (c URLConfig) OpenBucket(ctx context.Context) (*blob.Bucket, error) {
 	if err != nil {
 		return nil, fmt.Errorf("open bucket by URL: %v", err)
 	}
-
 	return b, nil
 }