start cleaning up repl/debugger error handling

start cleaning up repl/debugger error handling

* module/system/repl/repl.scm (prompting-meta-read): Catch and print
  read errors here, returning unspecified in that case.
  (start-repl): Don't enable the debugger while reading expressions.
  Adapt with-backtrace to with-error-handling.

* module/system/vm/debug.scm (run-debugger, debugger-repl): No need to
  take a stack, the frames vector is sufficient.
  (call-with-error-handling, with-error-handling): New public utilities.
  Notably they do not poke the-last-stack.