merge_rft_ertobs: validate OBSDIR input

[alifbe]

Ensure that OBSDIR exist and trigger exception otherwise.

Solve #559

[codecov-commenter]

Codecov Report

Merging #616 (46a9988) into main (de57617) will increase coverage by 0.00%.
Report is 2 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 46a9988 differs from pull request most recent head 46bcb37. Consider uploading reports for the commit 46bcb37 to get more accurate results

@@           Coverage Diff           @@
##             main     #616   +/-   ##
=======================================
  Coverage   84.72%   84.72%           
=======================================
  Files          52       52           
  Lines        7507     7510    +3     
=======================================
+ Hits         6360     6363    +3     
  Misses       1147     1147           

Files	Coverage Δ
src/subscript/eclcompress/allowlist.py	100.00% <ø> (ø)
src/subscript/merge_rft_ertobs/merge_rft_ertobs.py	94.59% <100.00%> (+0.22%)	⬆️

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[rnyb]

LGTM