gpg: New function to printed a detailed error code.

* g10/misc.c (print_reported_error): New.

Often the user is only interested in a catch all error code like "not
found" but sometimes it is useful to also see the real reason. By
this function this can easily be achieved. Example:

err = search_for_key (keyid)
if (err)
  {
    log_info ("error locating key '%s': %s\n",
              keyid, gpg_strerror (GPG_ERR_NOT_FOUND));
    print_reported_error (err, GPG_ERR_NOT_FOUND);
  }

results in

gpg: error locating key 'foobar': not found
gpg: (reported error: no keyring <keybox>)

where the second line is only printed in verbose mode and if ERR is
not GPG_ERR_NOT_FOUND.

• Signed-off-by: Werner Koch <wk@gnupg.org>