Fixed unwanted error propagation in FindBob

FindBob.cmake

@@ -119,6 +119,7 @@ function(find_bob_package package)
 
   # assert that we have found the package
   if (NOT _package_dir)
+    unset(_package_dir CACHE)
     if (Bob_FIND_REQUIRED)
       # if the package was required and we didn't find it, stop here
       message(FATAL_ERROR "Could not find bob package '${package}'")