When in batch mode, `debug-on-error' can be used to make Emacs
print backtraces of any errors that occur:

    $ emacs --batch --eval "(let ((debug-on-error t)) (error \"...\"))"
    Error: (error (error "..."))
      (signal error ("..."))
      (error "...")
      (let ((debug-on-error t)) (error "..."))
      (eval (let ((debug-on-error t)) (error "...")) t)
      (command-line-1 ("--eval" "(let ((debug-on-error t)) (error \"...\"))"))
      (command-line)
      (normal-top-level)
    ...

However, variable `inhibit-message' affects it:

    $ emacs --batch --eval "(let ((debug-on-error t) (inhibit-message
t)) (error \"...\"))"
    ...

This is wrong.  When binding `inhibit-message' you have no idea
that an error might happen inside the block.  Actually, you
rather hope it doesn't.  In other words, when binding
`inhibit-message' you never target error backtraces -- those are
only unwanted side-effects of bugs in your code.

Attached patch makes function `debug' temporarily rebind
`inhibit-message' to nil. The patch is just three lines, but `diff'
got mad because of reindentation.

Paul