Return failure on zip error.

src/main.cpp

@@ -68,7 +68,7 @@ int main(int argc, char *argv[])
 
     if (vm.count("input-file") > 0)
     {
-        cout << "\nINPUT FILES: ";
+        cout << "\nINPUT FILES:\n";
         for (const auto &file : vm["input-file"].as<std::vector<std::string>>())
         {
             cout << "  " << file << ":\n";
@@ -83,6 +83,7 @@ int main(int argc, char *argv[])
             {
                 cerr << translate("ERROR: ") << e.what() << '\n';
                 cerr << translate("Error while processing EPUB file.") << '\n';
+                return EXIT_FAILURE;
             }
         }
     }