ci: unmute submodule clone on appveyor

tools/appveyor/install.ps1

@@ -1,8 +1,13 @@
-$ErrorActionPreference = "Stop"
+$ErrorActionPreference = "Continue"
 
 try {
     & git submodule sync
-    & git submodule --quiet update --init --recursive
+    & git submodule update --init --recursive
+
+    if ($LASTEXITCODE -and $LASTEXITCODE -ne 0) {
+        Write-Host -ForegroundColor Red "`n`n***Cloning submodules failed. Exiting ... ***"
+        exit $LASTEXITCODE
+    }
 
     Write-Host -ForegroundColor Green "`n### Installing sphinx ###`n"
     & cinst sphinx --source python