Provide standard errors for API.

api/errors.go

@@ -0,0 +1,24 @@
+// Copyright © 2023 Attestant Limited.
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package api
+
+import "errors"
+
+// ErrUnsupportedVersion is returned when the data requested is not present for the
+// version of the struct.
+var ErrUnsupportedVersion = errors.New("unsupported version")
+
+// ErrDataMissing is returned when the data requested is missing from the versioned
+// struct.
+var ErrDataMissing = errors.New("data missing")

api/errors_test.go

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+// Copyright © 2023 Attestant Limited.
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package api_test
+
+import (
+	"testing"
+
+	"github.com/attestantio/go-eth2-client/api"
+	"github.com/attestantio/go-eth2-client/spec"
+	"github.com/stretchr/testify/require"
+)
+
+func TestError(t *testing.T) {
+	v1 := &api.VersionedProposal{
+		Version: spec.DataVersionAltair,
+	}
+
+	_, err := v1.KZGProofs()
+	require.ErrorIs(t, err, api.ErrUnsupportedVersion)
+
+	v2 := &api.VersionedProposal{
+		Version: spec.DataVersionDeneb,
+	}
+
+	_, err = v2.KZGProofs()
+	require.ErrorIs(t, err, api.ErrDataMissing)
+}