Add option to crash autorestic when key is missing instead of generating a new key (#383)

* feat(backend): add requireKey option to backend

This option will prevent `autorestic` from generating a key and will
cause it to crash instead. This is intended for use cases where you want
to provision the key yourself and don't want `autorestic` to
accidentally generate one for you.

* doc(backend): document requireKey

internal/backend_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 
 	"github.com/spf13/viper"
+	"github.com/stretchr/testify/assert"
 )
 
 func TestGenerateRepo(t *testing.T) {
@@ -222,4 +223,16 @@ func TestValidate(t *testing.T) {
 		}
 		assertEqual(t, err.Error(), "Backend \"foo\" has no \"path\"")
 	})
+
+	t.Run("require key with no key", func(t *testing.T) {
+		b := Backend{
+			name:       "foo",
+			Type:       "local",
+			Path:       "~/foo/bar",
+			RequireKey: true,
+		}
+		err := b.validate()
+		fmt.Printf("error: %v\n", err)
+		assert.EqualError(t, err, "backend foo requires a key but none was provided")
+	})
 }