ssh: raise clear error when broken symbolic link is found

[njzjz]

One currently gets a RecursionError when a broken symbolic link is uploaded, as it tries to walk again and again.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.07% ⚠️

Comparison is base (cc08915) 57.75% compared to head (c9bb297) 57.69%.

❗ Current head c9bb297 differs from pull request most recent head 650d6cc. Consider uploading reports for the commit 650d6cc to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #366      +/-   ##
==========================================
- Coverage   57.75%   57.69%   -0.07%     
==========================================
  Files          30       30              
  Lines        3371     3373       +2     
==========================================
- Hits         1947     1946       -1     
- Misses       1424     1427       +3     

Files Changed	Coverage Δ
dpdispatcher/ssh_context.py	80.80% <0.00%> (-0.39%)	⬇️

... and 1 file with indirect coverage changes

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