feat: notifying incosistent fs version problem with exit code

[PerseidMeteor]

If acceld converts with different fs version cache, leading to an inconsistent fs version problem when merging into boostrap layer. So we need to notify acceld that an inconsistent version occured and handle this error.

Relevant Issue (if applicable)

If there are Issues related to this PullRequest, please list it.

Details

Define InconsistentFsVersion with thiserror and check error type after merge, if is fs version inconsistent problem, just return status code 2.

Types of changes

What types of changes does your PullRequest introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation Update (if none of the other choices apply)

Checklist

Go over all the following points, and put an x in all the boxes that apply.

• I have updated the documentation accordingly.
• I have added tests to cover my changes.

[codecov]

Codecov Report

Merging #1424 (a4e40a3) into master (b777564) will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #1424      +/-   ##
==========================================
- Coverage   46.44%   46.43%   -0.01%     
==========================================
  Files         123      123              
  Lines       38702    38705       +3     
  Branches    38702    38705       +3     
==========================================
- Hits        17974    17973       -1     
- Misses      19755    19759       +4     
  Partials      973      973              

Files Changed	Coverage Δ
src/bin/nydus-image/main.rs	41.58% <0.00%> (-0.20%)	⬇️
rafs/src/metadata/mod.rs	52.49% <44.00%> (+0.20%)	⬆️

... and 3 files with indirect coverage changes

[imeoer]

LGTM! Thanks!