For future reference:
- -3 means the console was unable to detect that an SD card was inserted, or that the SD card has been formatted with an unsupported filesystem
- -4 means that the console detected an SD card and attempted to mount it, but failed, possibly due to filesystem corruption errors
- -5 means that the SD card was successfully mounted, but the program to launch could not be found in the right location on the SD card
If you're really getting all three of these errors intermittently, I would suggest trying a different SD card entirely, if you have one. It sounds to me like there is a hardware fault in your card or something along those lines.
If you're absolutely sure the SD card is formatted correctly (FAT32 filesystem with 32KB allocation clusters, I've heard that Windows users have a lot of luck using GUIFormat for this) and that the Homebrew Launcher is in the right location on your card (/wiiu/apps/homebrew_launcher/homebrew_launcher.elf) then I would definitely suggest trying a different card.