Improve debug output of TestFileExists

test/TestFileExists.cmake

@@ -9,5 +9,19 @@ endif()
 set(FILE_PATH "${CMAKE_ARGV3}")
 
 if(NOT ( EXISTS "${FILE_PATH}" ))
-    message(FATAL_ERROR "Test failed: File `${FILE_PATH}` does not exist.")
+    set(error_details "File `${FILE_PATH}` does not exist!\n")
+    set(PARENT_TREE "${FILE_PATH}")
+    cmake_path(HAS_PARENT_PATH PARENT_TREE has_parent)
+    while(has_parent)
+        cmake_path(GET PARENT_TREE PARENT_PATH PARENT_TREE)
+        cmake_path(HAS_PARENT_PATH PARENT_TREE has_parent)
+        if(EXISTS "${PARENT_TREE}")
+            file(GLOB dir_contents LIST_DIRECTORIES true "${PARENT_TREE}/*")
+            list(APPEND error_details "Found Parent directory `${PARENT_TREE}` exists and contains:\n" ${dir_contents})
+            break()
+        else()
+            list(APPEND error_details "Parent directory `${PARENT_TREE}` also does not exist!")
+        endif()
+    endwhile()
+    message(FATAL_ERROR "Test failed: ${error_details}")
 endif()