client: make error a bit more helpful

[Rubusch]

If a Place has multiple SSH targets, typing "labgrid-client ssh " triggers an ambiguous "multiple targets" error. It's usually just a missing -n flag, so let's point the user in the right direction with a clearer hint in the logs.

---

I'm not sure if you could add this? IMHO the "multiple targets" as only error message can be a bit confusing. You may lose quite some time double-checking configs, for actually a subtle commandline mistake which easily can be identified at
the client, and just a little note like this could save you some time. Then, the next colleague comes up with the same issue again, and again.

Once you know how it works, you never will miss a "-n" again, and for sure this is not the biggest issue. I understand
all this and no problem if you drop this commit. No problem! Anyway, just think about it ;)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.7%. Comparing base (bfb6ff2) to head (429160d).
⚠️ Report is 11 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/remote/client.py	0.0%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1844     +/-   ##
========================================
- Coverage    46.7%   46.7%   -0.1%     
========================================
  Files         179     179             
  Lines       14202   14205      +3     
========================================
  Hits         6645    6645             
- Misses       7557    7560      +3     

Flag	Coverage Δ
3.10	?
3.11	46.7% <0.0%> (-0.1%)	⬇️
3.12	46.7% <0.0%> (-0.1%)	⬇️
3.13	46.7% <0.0%> (-0.1%)	⬇️
3.14	46.7% <0.0%> (-0.1%)	⬇️

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