Pages with a StandardController expose a property:

`{!Messages}`

Without the need for a custom controller or extensions, can do things like:

    <apex:pageBlock rendered="{!ISNULL(Messages)}">
        <!-- detailed troubleshooting instructions -->
    </apex:pageBlock>

    <apex:pageBlock rendered="{!NOT(ISNULL(Messages))}">
        <!-- normal page functionality -->
    </apex:pageBlock>