Update warning message if root certs were not able to be updated

[#171222936](https://www.pivotaltracker.com/story/show/171222936)
 As a platform operator I should see a clear error message when stembuild can't access the windows update server to update the root certs.

Co-authored-by: Flora Gallina-Jones <fgallinajones@pivotal.io>
Co-authored-by: Austina Lin <aulin@pivotal.io>

stemcell-automation/AutomationHelpers.ps1

@@ -499,17 +499,8 @@ function Extract-LGPO
 
 function Install-WUCerts
 {
-    try
-    {
         Get-WUCerts
         Write-Log "Successfully retrieved Windows Update certs"
-    }
-    catch [Exception]
-    {
-        Write-Log $_.Exception.Message
-        Write-Log "Failed to retrieve updated root certificates from the public Windows Update Server."
-        throw $_.Exception
-    }
 }
 
 function Create-VersionFile

stemcell-automation/ProvisionVM.ps1

@@ -21,7 +21,7 @@ function ProvisionVM() {
     catch [Exception]
     {
         Write-Log $_.Exception.Message
-        Write-Warning "This should not impact the successful execution of stembuild construct. If the root certificates are out of date, Diego cells running on VMs built off of this stemcell may not be able to make outbound network connections."
+        Write-Warning "Failed to retrieve updated root certificates from the public Windows Update Server. This should not impact the successful execution of stembuild construct. If your root certificates are out of date, Diego cells running on VMs built from this stemcell may not be able to make outbound network connections."
     }
     Create-VersionFile -Version $Version
     Restart-Computer