handle message unknown charset error

This change handles message parse errors by printing the error when the
user tries to view the message. Specifically only handling unknown
charset errors in this patch, but there are many types of invalid
messages that can be handled in this way.

aerc currently leaves certain messages in the msglist in the pending
(spinner) state, and I'm unable to view or modify the message. aerc also
only prints parse errors with message when they are initially loaded.
This UX is a little better, because you can still see the header info
about the message, and if you try to view it, you will see the specific
error.

worker/lib/parse_test.go

@@ -0,0 +1,65 @@
+package lib
+
+import (
+	"bytes"
+	"io"
+	"io/ioutil"
+	"path/filepath"
+	"testing"
+
+	"git.sr.ht/~sircmpwn/aerc/models"
+)
+
+func TestMessageInfoHandledError(t *testing.T) {
+	rootDir := "testdata/message/invalid"
+	msgFiles, err := ioutil.ReadDir(rootDir)
+	die(err)
+
+	for _, fi := range msgFiles {
+		if fi.IsDir() {
+			continue
+		}
+
+		p := fi.Name()
+		t.Run(p, func(t *testing.T) {
+			m := newMockRawMessageFromPath(filepath.Join(rootDir, p))
+			mi, err := MessageInfo(m)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if perr := mi.Error; perr == nil {
+				t.Fatal("Expected MessageInfo.Error, got none")
+			}
+		})
+	}
+}
+
+type mockRawMessage struct {
+	body []byte
+}
+
+func newMockRawMessage(body []byte) *mockRawMessage {
+	return &mockRawMessage{
+		body: body,
+	}
+}
+
+func newMockRawMessageFromPath(p string) *mockRawMessage {
+	b, err := ioutil.ReadFile(p)
+	die(err)
+	return newMockRawMessage(b)
+}
+
+func (m *mockRawMessage) NewReader() (io.Reader, error) {
+	return bytes.NewReader(m.body), nil
+}
+func (m *mockRawMessage) ModelFlags() ([]models.Flag, error) { return nil, nil }
+func (m *mockRawMessage) Labels() ([]string, error)          { return nil, nil }
+func (m *mockRawMessage) UID() uint32                        { return 0 }
+
+func die(err error) {
+	if err != nil {
+		panic(err)
+	}
+}