Add `define-bash-function` macro

Add `define-bash-function` macro

  * lisp/gnu/bash.scm(bind-scm-function): Catch exceptions here, not in
  C code to more fine-grained control of return value

  * lisp/gnu/bash.scm(define-bash-function): New macro, defining
  scheme procedure and binding bash function of same name to it.

  * tests/command.bash.in: test wrong-type-argument errors

  * tests/command.scm.in: Use `define-bash-function` in tests