feat(webp): add partial success to conversion

So we only keep images that couldn't be optimized and return the chapter

converter/errors/converter_errors.go

@@ -0,0 +1,13 @@
+package errors
+
+type PageIgnoredError struct {
+	s string
+}
+
+func (e *PageIgnoredError) Error() string {
+	return e.s
+}
+
+func NewPageIgnored(text string) error {
+	return &PageIgnoredError{text}
+}