feat: allow checking multiple errors codes in IsError (#491)

Useful when one need to check multiple error codes at once, without
having to cast every time.

```diff
-hcloud.IsError(err, hcloud.ErrorCodeConflict) || hcloud.IsError(err, hcloud.ErrorCodeLocked)
+hcloud.IsError(err, hcloud.ErrorCodeConflict, hcloud.ErrorCodeLocked)
```

hcloud/error.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"slices"
 )
 
 // ErrorCode represents an error code returned from the API.
@@ -126,11 +127,11 @@ type ErrorDetailsInvalidInputField struct {
 	Messages []string
 }
 
-// IsError returns whether err is an API error with the given error code.
-func IsError(err error, code ErrorCode) bool {
+// IsError returns whether err is an API error with one of the given error codes.
+func IsError(err error, code ...ErrorCode) bool {
 	var apiErr Error
 	ok := errors.As(err, &apiErr)
-	return ok && apiErr.Code == code
+	return ok && slices.Index(code, apiErr.Code) > -1
 }
 
 type InvalidIPError struct {

hcloud/error_test.go

@@ -78,7 +78,7 @@ func TestError_Error(t *testing.T) {
 func TestIsError(t *testing.T) {
 	type args struct {
 		err  error
-		code ErrorCode
+		code []ErrorCode
 	}
 	tests := []struct {
 		name string
@@ -89,46 +89,54 @@ func TestIsError(t *testing.T) {
 			name: "hcloud error with code",
 			args: args{
 				err:  Error{Code: ErrorCodeUnauthorized},
-				code: ErrorCodeUnauthorized,
+				code: []ErrorCode{ErrorCodeUnauthorized},
+			},
+			want: true,
+		},
+		{
+			name: "hcloud error with many codes",
+			args: args{
+				err:  Error{Code: ErrorCodeUnauthorized},
+				code: []ErrorCode{ErrorCodeUnauthorized, ErrorCodeConflict},
 			},
 			want: true,
 		},
 		{
 			name: "hcloud error with different code",
 			args: args{
 				err:  Error{Code: ErrorCodeConflict},
-				code: ErrorCodeUnauthorized,
+				code: []ErrorCode{ErrorCodeUnauthorized},
 			},
 			want: false,
 		},
 		{
 			name: "wrapped hcloud error with code",
 			args: args{
 				err:  fmt.Errorf("wrapped: %w", Error{Code: ErrorCodeUnauthorized}),
-				code: ErrorCodeUnauthorized,
+				code: []ErrorCode{ErrorCodeUnauthorized},
 			},
 			want: true,
 		},
 		{
 			name: "wrapped hcloud error with different code",
 			args: args{
 				err:  fmt.Errorf("wrapped: %w", Error{Code: ErrorCodeConflict}),
-				code: ErrorCodeUnauthorized,
+				code: []ErrorCode{ErrorCodeUnauthorized},
 			},
 			want: false,
 		},
 		{
 			name: "non-hcloud error",
 			args: args{
 				err:  fmt.Errorf("something went wrong"),
-				code: ErrorCodeUnauthorized,
+				code: []ErrorCode{ErrorCodeUnauthorized},
 			},
 			want: false,
 		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			assert.Equalf(t, tt.want, IsError(tt.args.err, tt.args.code), "IsError(%v, %v)", tt.args.err, tt.args.code)
+			assert.Equalf(t, tt.want, IsError(tt.args.err, tt.args.code...), "IsError(%v, %v)", tt.args.err, tt.args.code)
 		})
 	}
 }