A more informative error for  #423
A more informative error for  #423

Users might commonly save with a non-default metric, then try to open using the default metric. This gives a more informative message.

@erikbern let me know if you like the language and if line length is a concern.
eddelbuettel/annoy,spotify/annoy,eddelbuettel/annoy,spotify/annoy,eddelbuettel/annoy,eddelbuettel/annoy,spotify/annoy,spotify/annoy