/// <summary>
/// A custom editor part for picking data fields for a table web part
/// </summary>
class CustomEditorPart:EditorPart
{
#region Declarations

// In addition to other controls, etc, declare an error variable

string _errorText = string.Empty;

#endregion Declarations

#region Apply Changes

/// <summary>
/// Applies the changes to the web part properties
/// </summary>
/// <returns></returns>
public override bool ApplyChanges()
{
try
{

// You code goes here

return true;
}
catch (Exception ex)
{
_errorText = ex.Message;
return false;
}
}

#endregion Apply Changes

#region Rendering (Error Text)

/// <summary>
/// Update the error text if needed
/// </summary>
/// <param name="e"></param>
protected override void OnPreRender(EventArgs e)
{
if (_errorText != string.Empty)
{
this.Zone.ErrorText += Environment.NewLine + _errorText;
}

base.OnPreRender(e);
}

#endregion Rendering (Error Text)
}