bootstrap.sh: fails if there is no entry for a tool os combination (#19640)

scripts/bootstrap.sh

@@ -165,8 +165,8 @@ fetchTool()
     xmlFileAsString=`cat "$vcpkgRootDir/scripts/vcpkgTools.xml"`
     toolRegexStart="<tool name=\"$tool\" os=\"$os\">"
     toolData="$(extractStringBetweenDelimiters "$xmlFileAsString" "$toolRegexStart" "</tool>")"
-    if [ "$toolData" = "" ]; then
-        echo "Unknown tool: $tool"
+    if [ "$toolData" = "" ] || [[ "$toolData" == "<?xml"* ]]; then
+        echo "No entry for $toolRegexStart in $vcpkgRootDir/scripts/vcpkgTools.xml"
         return 1
     fi