index: Avoid async shutdown on init error #21796

An async shutdown during init is confusing when a simple boolean return value can be used for a synchronous shutdown.

This also changes the error message on stderr from:

Error: A fatal internal error occurred, see debug.log for details
Error: A fatal internal error occurred, see debug.log for details

To:

Error: basic block filter index best block of the index goes beyond pruned data. Please disable the index or reindex (which will download the whole blockchain again)

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #21866 ([Bundle 7/7] validation: Farewell, global Chainstate! by dongcarl)
• #21767 ([Bundle 6/n] Prune g_chainman usage in auxiliary modules by dongcarl)
• #19064 (refactor: Cleanup thread ctor calls by hebasto)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Code review ACK—however this introduces two warnings here locally:

…/bitcoin/src/test/coinstatsindex_tests.cpp:35:5: warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]                                                       
    coin_stats_index.Start();                                                                                     
    ^~~~~~~~~~~~~~~~~~~~~~                          
1 warning generated.            
…/bitcoin/src/init.cpp:1566:9: warning: ignoring return value of function declared with 'nodiscard' attribute [-Wunused-result]                                                                          
        g_coin_stats_index->Start();                   
        ^~~~~~~~~~~~~~~~~~~~~~~~~                
1 warning generated.  

Rebased due to change in adjacent line

Code review ACK faad68fcd440e77e61a5a1560471417dd984e390