On a production site, with all the debugging turned off, most errors will be hidden from you and the Moodle users. 

But when you do have a problem, it can be helpful to see any error messages. (If you know how, you can also access the error from the server error log.)

Turn error reporting up to max, and send errors to browser

Visit : site admin -> development -> debugging 

Once there choose:

  • From "Debugging" dropdown
    "Developer: Extra Moodle debug messages for developers"

  • Display debug messages

Just remember to turn Debugging down to "None: Do not show any errors or warnings" when you are finished trouble shooting. It is best not to scare true users, or give any extra information to bad ones.