Send a backtrace for the error error to output.

The keywords arguments are:

* :output - where to send the output. This can be:

* a string (which is assumed to designate a pathname)
* an open stream
* nil to indicate that the backtrace information should be
returned as a string

* if-exists - what to do if output designates a pathname and
the pathname already exists. Defaults to :append.

* verbose - if true, then a message about the backtrace is sent
to *terminal-io*. Defaults to nil.

If the output is nil, the returns the backtrace output as a
string. Otherwise, returns nil.