Fix screenshot crash with long video title, iina#3334

This commit will:

- Change the method  MPVController.handleEvent to raise an alert if
    screenshot generation failed
- Change the method  Utility.getLatestScreenshot to guard against an empty
    screenshot directory

This commit adds new text for the alert that will require localization.

iina/Base.lproj/Localizable.strings

@@ -177,6 +177,8 @@
 
 "alert.playlist.error_deleting" = "Error deleting: %@";
 
+"alert.screenshot.error_taking" = "Cannot take a screenshot!";
+
 "alert.sub.no_selected" = "Please select a downloaded subtitle first.";
 "alert.sub_lang_not_set" = "Please set preferred subtitle languages in preferences before using OpenSubtitles. English(eng) will be used this time.";
 "alert.sub.cannot_save_passwd" = "Cannot save your password to Keychain: %@";

iina/MPVController.swift

@@ -672,6 +672,18 @@ class MPVController: NSObject {
     case MPV_EVENT_COMMAND_REPLY:
       let reply = event.pointee.reply_userdata
       if reply == MPVController.UserData.screenshot {
+        let code = event.pointee.error
+        guard 0 <= code else {
+          let error = String.init(cString: mpv_error_string(code))
+          Logger.log("Cannot take a screenshot, mpv API error: \(error), Return value: \(code)", level: .error)
+          // Unfortunately the mpv API does not provide any details on the failure. The error
+          // code returned maps to "error running command", so all the alert can report is
+          // that we cannot take a screenshot.
+          DispatchQueue.main.async {
+            Utility.showAlert("screenshot.error_taking")
+          }
+          return
+        }
         player.screenshotCallback()
       }
 

iina/Utility.swift

@@ -461,7 +461,7 @@ class Utility {
     guard let contents = try? FileManager.default.contentsOfDirectory(
       at: folder,
       includingPropertiesForKeys: [.creationDateKey],
-      options: .skipsSubdirectoryDescendants) else { return nil }
+      options: .skipsSubdirectoryDescendants), !contents.isEmpty else { return nil }
 
     var latestDate = Date.distantPast
     var latestFile: URL = contents[0]

iina/en.lproj/Localizable.strings

@@ -177,6 +177,8 @@
 
 "alert.playlist.error_deleting" = "Error deleting: %@";
 
+"alert.screenshot.error_taking" = "Cannot take a screenshot!";
+
 "alert.sub.no_selected" = "Please select a downloaded subtitle first.";
 "alert.sub_lang_not_set" = "Please set preferred subtitle languages in preferences before using OpenSubtitles. English(eng) will be used this time.";
 "alert.sub.cannot_save_passwd" = "Cannot save your password to Keychain: %@";