DAOS-19151 container: refine some logs in IV and EC agg path

[gnailzenh]

1. lower the log level when container IV hit -DER_CONT_NONEXIST
2. refine err code returned when abort EC agg

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'Enormous DER_CONT_NONEXIST errors noticed on server logs while rebuild and IOR running in parallel.'
Status is 'Awaiting backport'
Labels: 'test_2.6.5p1,test_2.8.0rc'
Job should run at elevated priority (1)
https://daosio.atlassian.net/browse/DAOS-19151