errors(postgresql): Clarify missing ledger state delta message.

[winder]

Summary

If the postgresql exporter is given a block without a ledger state delta object, the error message gave no indication about what the user should do. Attempt to clarify the situation by telling the user to check that their algod importer is in follower mode.

Test Plan

Update existing unit test.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@f5c2701). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master       #6   +/-   ##
=========================================
  Coverage          ?   66.64%           
=========================================
  Files             ?       32           
  Lines             ?     1805           
  Branches          ?        0           
=========================================
  Hits              ?     1203           
  Misses            ?      535           
  Partials          ?       67           

Impacted Files	Coverage Δ
...lugins/exporters/postgresql/postgresql_exporter.go	78.20% <100.00%> (ø)

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