Fix error message for planned reparent shard

[GuptaManan100]

Description

This PR fixes the issue described in #15380.
In PlannedReparentShard we now give a more descriptive error message telling exactly why the tablets cannot be promoted. Post these changes, the error message now looks like -

cannot find a tablet to reparent to
zone1-0000000100 does not match the new primary alias provided
zone1-0000000101 does not match the new primary alias provided
zone2-0000000102 is not a replica
zone1-0000000103 is not in the same cell as the previous primary
zone1-0000000104 has 1m40s replication lag which is more than the tolerable amount

Related Issue(s)

• Fixes Bug Report: Reparent error message ambiguous #15380

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

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[mattlord]

❤️

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.72%. Comparing base (696fe0e) to head (6693c85).
Report is 136 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15529      +/-   ##
==========================================
- Coverage   67.41%   65.72%   -1.70%     
==========================================
  Files        1560     1560              
  Lines      192752   194552    +1800     
==========================================
- Hits       129952   127875    -2077     
- Misses      62800    66677    +3877     

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