Misc fixes

[abhidg]

• Report carbon intensity in human readable format #92
• Report carbon intensity in human-readable format #92
• Show friendlier error when at(1) is missing
• Show error if duration is greater than 48 hours

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 87.50000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 89.19%. Comparing base (8e80090) to head (5af7c94). Report is 6 commits behind head on main.

Files	Patch %	Lines
cats/__init__.py	82.35%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #94 +/- ## ========================================== - Coverage 89.22% 89.19% -0.04% ========================================== Files 14 14 Lines 529 546 +17 ========================================== + Hits 472 487 +15 - Misses 57 59 +2 ```

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