Improve the error message when collecting restart files.

[Qian-HuiChen]

Make the FileNotFound error message clearer when collecting restart files.

Closes #705.
Ref issue in OM3 config repo.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.72%. Comparing base (d97ecfa) to head (c6daca3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #708      +/-   ##
==========================================
+ Coverage   63.70%   63.72%   +0.02%     
==========================================
  Files          67       67              
  Lines        5554     5555       +1     
==========================================
+ Hits         3538     3540       +2     
+ Misses       2016     2015       -1     

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[Qian-HuiChen]

@manodeep @anton-seaice thanks for raising the issue!
This PR aims to make payu deliver a clearer error message to the users. You feedback/comments/suggestions are highly appreciated 🙂
Edit:
The current error message will be:

::
File "/home/.../cesm_cmeps.py", line 336, in _collect_restart_files
    raise FileNotFoundError(
FileNotFoundError:
Payu Error: Restart pointer file not found at the end of payu run: /scratch/.../100km_jra_ryf/rpointer.cpl
Possible cause: model runtime is shorter than the restart write frequency.
Fix:
    1. Remove the incomplete restart subdirectory in the archive path
    2. Adjust the restart write frequency to be shorter than/equal to the model runtime.

[anton-seaice]

Looks good - could you add a test in

https://github.com/ACCESS-NRI/payu/blob/705-om3-errormsg/test/models/access-om3/test_access_om3.py

[anton-seaice]

Suggested change

	2. Adjust the restart write frequency to be shorter than/equal to the model runtime."""
	2. Adjust the restart write frequency to write a restart at the end of the model run."""

[Qian-HuiChen]

Sounds good. Unit test is added to confirm error message is delivered when

1. rpointer file is missing
2. restart files in pointer are missing