[SalesForce] Required Error Message in Visualforce Page

You can overwrite the standard salesforce error message css classes. In my example i have a simple error message with only short summary and a custom icon. So i took all error css classes and changed the icon. Then i have wrapped my message in a outputPanel and edited the message class (but only for this output panel): removed background and border. You can pack that two css classes to the static resource and include on your pages. Don't forget to wrap the custom message into the output pannel:

<style>
.errorM4 .msgIcon, .errorSmall {
    background-image: url(/img/samples/flag_red.gif) !important;
}

.myCustomMessage .message {
    background: none  !important;
    border: none !important;
}

</style>

    <apex:outputPanel styleClass="myCustomMessage">
        <apex:pageMessage severity="error"
                          strength="1"
                          summary="Some error text" />
    </apex:outputPanel>

Standard error message layout:

Customized error message:

You need to rerender pageMessages section by commandButton's attribute

<apex:pageBlock>
    <apex:pageMessages id="msgId"/>


    <apex:commandButton name="Submit" action="{!exchangeInv}" rerender ="msgId"/>
</apex:pageBlock>

Take this approach:

Since you are throwing applicationException, so better to use throw new applicationException and in the catch block, wrap that message into pageMessages

public PageReference exchangeInv() 
{
    try
    {
        if (opp.StageName == 'Contract Executed') {
        // do some stuff
        PageReference pageRef = new PageReference('/'+opp.Id);
        return pageRef;
      }
      else {
        throw new applicationException('error');
        return null;
      }
    }catch (Exception ex)
    {
        ApexPages.Message msg = new ApexPages.Message(ApexPages.Severity.Error, ex.getMessage());
        ApexPages.addMessage(msg);
        return null;
    }  
}

or, if you don't want to use applicationException then use the error into pageMessages as follows:

public PageReference exchangeInv() 
{

    if (opp.StageName == 'Contract Executed') {
        // do some stuff
        PageReference pageRef = new PageReference('/'+opp.Id);
        return pageRef;
      }
      else {
        ApexPages.Message msg = new ApexPages.Message(ApexPages.Severity.Error, 'error');
        ApexPages.addMessage(msg);
        return null;
      }

}