ui: 'with-error-handling' does not unwind the stack.

ui: 'with-error-handling' does not unwind the stack.

Since a07d5e558b5403dad0a59776b950b6b02169c249, we've been getting
useless backtraces upon unhandled errors, like this:

  Backtrace:
	     1 (primitive-load "/home/…/bin/guix")
  In guix/ui.scm:
    1953:12  0 (run-guix-command _ . _)

  guix/ui.scm:1953:12: In procedure run-guix-command:
  In procedure struct-vtable: Wrong type argument in position 1 (expecting struct): #f

This change finally gives us real backtraces back.

* guix/ui.scm (guard*): New macro.
(call-with-error-handling): Use it instead of 'guard'.