Singer-songwriter Dan Bern. (Photo by Todd Adamson.) As Dan Bern explains it, when someone has a story to tell in Nashville, they write a nice little song that manages to tie everything together in two and a half minutes with a melody to boot. But in L.A., where Bern hangs his hat these days, when someone has a story, they write a movie. Movies cost a lot of dough, though — something most singer-songwriters…