fix(state): Replace a chain length assertion with a NotReadyToBeCommitted error

[teor2345]

Motivation

The auditors were concerned about the possibility of a panic in a state verification check.

Solution

Replace the assertion with a block verification error.

Review

This is a low-priority audit cleanup.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

Follow Up Work

We should change the tests that currently check for success to check for this error instead, and remove the test workaround.

[codecov]

Codecov Report

Merging #7072 (804c93a) into main (a6f35af) will increase coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7072      +/-   ##
==========================================
+ Coverage   77.52%   77.54%   +0.01%     
==========================================
  Files         310      310              
  Lines       41694    41694              
==========================================
+ Hits        32325    32330       +5     
+ Misses       9369     9364       -5     

[dconnolly]

lgtm!