Fixed import error message when extensions not built
From a quick glance at the newcomers slack channel this is the question people ask about most often. It looks like pandas.compat indirectly imports the _libs folder via util._dectorators grabbing `cache_readonly`, so our try...except never gets the chance to run.

It would be nice to add a test for this, not sure the cleanest way how
nice, thanks

I'd never seen this message 😄 Is `--force` the best advise to give though? Isn't it ok to just recommend the incremental build? Never used force. I can update that too Thanks @WillAyd 