==with response codes==
function errorHandler( xhr, status, data ) {
  if( status === 500 ) {
    reportMessage( 'There was problem with server' );
  } else if( status === 200 ) {
	  if( typeof data === 'string' ) {
	    data = JSON.stringify( data );
	  }
    // inspect the data and work out the problem
  }
}

$.ajax({
  url: '/api.php...',
  success: function( data, status, xhr ) {
    if( data.error ) {
      errorHandler( xhr, status, data );
    } else {
      // success handler
    }
  },
  error: errorHandler
})

==without response codes==
function errorHandler( xhr, status, data ) {
  data = JSON.stringify( data );
  if( status === 500 ) {
    reportMessage( 'There was problem with server' );
  } else if( status === 404 ) {
    reportMessage( 'response not found' );
  } else if( status === 400 ) {
    reportMessage( 'problem with parameters' );
	// dive into data to see what problem is
  }
}

$.ajax({
  url: '/api.php...',
  success: function( data, status, xhr ) {
    // success handler
  },
  error: errorHandler
})