Surface step name when reporting error

[dshemetov]

          True, though it would be nice if `Error in (function (name, value)  : ` said `Error in step_epi_ahead() :` instead.

Originally posted by @dajmcdon in #286 (comment)

Our input validation tells which argument had the error, but doesn't specify which step in the recipe. The user can get there with traceback(), but it would be nice if we could surface this by default.