BUG: Teach UntarFile to delete dir on error

When tarball extraction fails we should still cleanup the temporary
extraction directory.  Otherwise the next attempt will create a new
directory and the first one will never be removed.

Modules/UntarFile.cmake

@@ -59,6 +59,7 @@ execute_process(COMMAND ${CMAKE_COMMAND} -E tar ${args} ${filename}
   RESULT_VARIABLE rv)
 
 if(NOT rv EQUAL 0)
+  file(REMOVE_RECURSE "${ut_dir}")
   message(FATAL_ERROR "error: untar of '${filename}' failed")
 endif()