List help topics when invalid topic is requested

[n-st]

Currently, the available help topics are only listed manually, scattered throughout different parts of the documentation.
It would be helpful to have an option for borg help that lists all available topics (e.g. borg help --list-topics).

Until that's implemented, this commit adds a list of available topics to the "topic not found" error message.
This might also remain useful even after the separate flag has been added. Alternatively, we could replace the list with a mention of the new --list-topics once that exists.

[ThomasWaldmann]

looks good.

[enkore]

Topics should probably be listed in borg [--]help, just like commands. This may be hacky, though, with argparse.

[codecov-io]

Codecov Report

Merging #3088 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3088      +/-   ##
==========================================
- Coverage   85.83%   85.79%   -0.04%     
==========================================
  Files          36       36              
  Lines        8943     8948       +5     
  Branches     1482     1482              
==========================================
+ Hits         7676     7677       +1     
- Misses        856      861       +5     
+ Partials      411      410       -1

Impacted Files	Coverage Δ
src/borg/archiver.py	86.03% <0%> (-0.27%)	⬇️
src/borg/archive.py	83.7% <0%> (+0.07%)	⬆️

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[ThomasWaldmann]

I agree with @enkore, that would be a better place (and also less strange than having to request an invalid topic just to see the list of topics). @n-st can you try to get it in there?

[ThomasWaldmann]

@n-st did you read @enkore's and my feedback?

[n-st]

Apologies for not replying earlier.
I did read your feedback, but didn't have time to pursue this beyond the quick'n'dirty fix I already pushed.
Not sure when I'll find time to work on this, so this improvement is effectively up for grabs by anyone who would like to implement it.

[ThomasWaldmann]

I've looked at it again.

We need both:

• the changes in this changeset (to be more helpful than "no help available on")
• the change @enkore suggested

[ThomasWaldmann]

thanks!