First of all, thanks so much for the blog reference. There was a lot of pieces there that I had not seen in terms of how to use certain VF components. For example,

ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, 'Ooops!'));

I didn't get from the VF Guide that this is how you add your own message. I don't think they have this line anywhere in the guide examples and I guess we're just supposed to figure it out from what's in there. I didn't.

However, the issue I have is to produce an error message for something that isn't really an error, as far as the system is concerned. I have an inputCheckbox for each line in my table, and want to display a message if a user forgets to select a line when modifying it.The system just reRenders the outputPanel with no changes, but I would like to display a message to the user reminding him to select a line. I would think maybe an actionSupport on the Create button, or possibly trying to use the onclick="" attribute could work, however I can't seem to use a boolean function to inform the attribute-not sure why.

For example:

 <apex:commandButton 
     action="{!IF(selectedCheck, createQuote, alert('Please select an item'))}" 
                this doesn't work
     value="Create Quote" >
 </apex:commandButton>

//with 

public boolean checkSelection;

 public boolean getCheckSelection(list<LineQuote> LQs)
    {
    	boolean selectCheck = false;
    	for (LineQuote LQ :LQs)
    	{
    		if(LQ.Selected)
    		{
    			selectCheck = true;
    		}
    	}
        return checkSelection;
    }

This returns the following error message: 

                Error: Unknown property 'OpportunityStandardController.checkSelection' 

       not sure why as the property is clearly defined in the controller. This is an Opportunity extension, so maybe that has something to do with it- I don't think so but I don't really know.

Anyway, here's how I solved it.

if (checkSelection)
{
   for (Object O :objectList) 
   {
       ...selected item is found, code runs normally
   }
}
else
{
    for (Object O :objectList)  // I can't create a Validation Rule for an unselected inputBox, I create my own error
    {
        try{                    
            Account error;
            insert error;
        }
        catch (Exception e){
            e.setMessage('Please Select an item');  //change the error Message on the system exception
            ApexPages.addMessages(e); //send the edited message to the page
            break;     // not sure why, but apparently this process needs to
                       // break a loop
                       // otherwise the page just refreshes without the error message
}

I know there is an Exception class that can be extended but the guide's examples are rather sparse, or else I'm too dense to figure out how to use it from the guide. I tried several times to create a myException object but nothing worked right. So why not create a system error, change the error message and send it to the page? One exception's as good as another for sending a message to the page, as long as I'm creating it on purpose.

Its probably a stupid way to do it, but it works.

However, I'd like to see a good example of how to use the Exception class for this kind of purpose. Anything to help understand the Dev Guide.