Adjust wording of MissingExternalDependency error

[mih]

The new phrasing should better cover the case of an existing installation
that is nevertheless broken, and fails an import test.

Fixes #5729

[adswa]

This looks better than the previous message to me.

[codecov]

Codecov Report

Merging #5803 (43018f1) into master (0b2d882) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5803      +/-   ##
==========================================
+ Coverage   90.62%   90.64%   +0.02%     
==========================================
  Files         309      309              
  Lines       42392    42436      +44     
==========================================
+ Hits        38417    38468      +51     
+ Misses       3975     3968       -7     

Impacted Files	Coverage Δ
datalad/support/exceptions.py	85.12% <100.00%> (ø)
datalad/cmdline/tests/test_main.py	96.63% <0.00%> (-0.61%)	⬇️
datalad/__init__.py	86.70% <0.00%> (-0.56%)	⬇️
datalad/cmd.py	91.20% <0.00%> (-0.09%)	⬇️
datalad/core/distributed/push.py	92.49% <0.00%> (-0.03%)	⬇️
datalad/support/gitrepo.py	91.99% <0.00%> (-0.02%)	⬇️
datalad/nonasyncrunner.py	100.00% <0.00%> (ø)
datalad/interface/__init__.py	100.00% <0.00%> (ø)
datalad/cmdline/tests/test_helpers.py	100.00% <0.00%> (ø)
datalad/distributed/create_sibling_gitlab.py	74.67% <0.00%> (ø)
... and 5 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0b2d882...43018f1. Read the comment docs.

[mih]

Thx @adswa !