[suggestion] more meaningful error messages with 'dest=qubes-ssh//'

[t-gh-ctrl]

Currently the error messages are very generic and usually don't point to the cause of the error. Wyng users are "power users" so finding the cause of the error(s) is usually pretty easy - but maybe this could be improved?

Eg.

Using a destination qube that doesn't exist:

Wyng 0.8 beta release 20250121
Traceback (most recent call last):
  File "/usr/local/bin/wyng", line 5172, in <module>
    aset        = get_configs(options)    ; dest = aset.dest
                  ^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/bin/wyng", line 2242, in get_configs
    dest.detect_state(opts.dedup, ro=opts.action not in write_actions)
  File "/usr/local/bin/wyng", line 2508, in detect_state
    do_exec([cmd])
  File "/usr/local/bin/wyng", line 2855, in do_exec
    return close()[-1]
           ^^^^^^^
  File "/usr/local/bin/wyng", line 2844, in close
    raise SPr.CalledProcessError([x for x in rclist if x][0], commands)
subprocess.CalledProcessError: Command '[['/usr/bin/qvm-run', '--no-color-stderr', '--no-color-output', '--nogui', '--no-filter-escape-chars', '-p', 'vm-backup-blah', 'set -e\nmkdir -p /tmp/wyngrpc']]' returned non-zero exit status 2.

Or anything that makes reaching the ssh folder impossible (eg. wrong ssh user, wrong host, wrong path)

Wyng 0.8 beta release 20250121
Traceback (most recent call last):
  File "/usr/local/bin/wyng", line 5172, in <module>
    aset        = get_configs(options)    ; dest = aset.dest
                  ^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/bin/wyng", line 2242, in get_configs
    dest.detect_state(opts.dedup, ro=opts.action not in write_actions)
  File "/usr/local/bin/wyng", line 2563, in detect_state
    if self.online and not self.dtmp or len(self.dtmp) != len(tmpprefix)+tmpdigits+3 \
                                        ^^^^^^^^^^^^^^
TypeError: object of type 'NoneType' has no len()

[tasket]

This is great feedback. Though in such cases you might use --debug if you're curious. I know that's not a general solution, but it can display more info about failed subprocesses if you have the stomach for it.