ease-of-use improvement to ,m; catch read errors at the repl

ease-of-use improvement to ,m; catch read errors at the repl

* module/system/repl/command.scm (module): Accept e.g. `,m ice-9 popen'
  in addition to `,m (ice-9 popen)'.

* module/system/repl/repl.scm (start-repl): Call read with a backtrace
  handler too.