BF: downloader - fail to download even on non-crippled FS if symlink exists

[yarikoptic]

Observed initially by @jwodder on
datalad/datalad-fuse#3 (comment)
since datalad-extensions-template default workflow runs on a crippled FS.

I think it makes sense to fail regardless either it is existing file or a
(broken) symlink since the idea is not to override a (possibly annexed) file

[codecov]

Codecov Report

Merging #5991 (bb7b17c) into maint (1e0f732) will decrease coverage by 2.01%.
The diff coverage is 100.00%.

❗ Current head bb7b17c differs from pull request most recent head 9144a00. Consider uploading reports for the commit 9144a00 to get more accurate results

@@            Coverage Diff             @@
##            maint    #5991      +/-   ##
==========================================
- Coverage   90.32%   88.31%   -2.02%     
==========================================
  Files         308      305       -3     
  Lines       42111    42098      -13     
==========================================
- Hits        38035    37177     -858     
- Misses       4076     4921     +845     

Impacted Files	Coverage Δ
datalad/distributed/tests/test_ria_basics.py	97.56% <100.00%> (-0.30%)	⬇️
datalad/downloaders/base.py	78.16% <100.00%> (ø)
datalad/nonasyncrunner.py	98.96% <100.00%> (-1.04%)	⬇️
datalad/tests/test_nonasyncrunner.py	100.00% <100.00%> (ø)
datalad/plugin/wtf.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/addurls.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/no_annex.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/add_readme.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/check_dates.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/support/tests/utils.py	0.00% <0.00%> (-100.00%)	⬇️
... and 72 more

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[yarikoptic]

crawler fails are likely unrelated: datalad/datalad-crawler#104 .

If no objections or other feedback -- will merge in a day or so. To make auto auto-mint release, might even merge with release label unless some other PR would "release" us sooner.

[mih]

LGTM! I will leave the merge to @yarikoptic to preserve the possibility to add the release tag.