Fix missing path debug

[amarkpayne]

I caught a typo in the messages the Arkane adapter returns when file paths are missing. While I was at it, I figured I would quickly refactor this code so that all missing file paths could be displayed if needed.

If you don't like the refactoring, I am fine with excluding all but the typo fix.

[codecov]

Codecov Report

Merging #395 into master will not change coverage.
The diff coverage is 11.11%.

@@           Coverage Diff           @@
##           master     #395   +/-   ##
=======================================
  Coverage   54.54%   54.54%           
=======================================
  Files          34       34           
  Lines       11904    11904           
  Branches     3633     3633           
=======================================
  Hits         6493     6493           
  Misses       4458     4458           
  Partials      953      953           

Impacted Files	Coverage Δ
arc/statmech/arkane.py	67.87% <11.11%> (ø)

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[xiaoruiDong]

Thanks for the contribution! The change makes sense and increases the robustness. I only had a minor comments:
At this line

ARC/arc/statmech/arkane.py

Line 401 in 2e71a1a

if freq_path:

which is in the same function, it will determine freq_path validation again, which I think is no longer needed. Can you take a look, and see if we can remove that if block? Thanks!

[alongd]

@amarkpayne, what is the status of this PR?