How can I disable or hide warnings and errors shown in the backend?
You actually should fix the error.

And since it's not a very good idea to globally disable all errors and warnings for all roles, please use Disable Messages to disable error messages based on configurable conditions.


  Gives a site owner options to disable specific messages shown to end
  users. The core Drupal message system as offered by drupal_set_message
  is an excellent way for modules to send out messages to the end users.
  However not all Drupal site owners are keen to show all the messages
  sent out by Drupal core and all modules to their users. This module
  gives site administrators a reasonably powerful way to filter out
  messages shown to the end users.
  
  Features
  
  
  Filter out messages that match a full text string exactly
  Filter out messages that match a regular expression.
  Permissions to specifically hide all messages of a given type from any role.
  Disable all filtering for specific users.
  Disable all filtering for specific paths.
  Apply filtering only for specific paths.
  Debug system to get messages in the HTML without showing it to the end users.