Merge bitcoin/bitcoin#27708: Return EXIT_FAILURE on post-init fatal errors

61c569ab60 refactor: decouple early return commands from AppInit (furszy)
4927167f85 gui: return EXIT_FAILURE on post-init fatal errors (furszy)
3b2c61e819 Return EXIT_FAILURE on post-init fatal errors (furszy)
3c06926cf2 refactor: index: use `AbortNode` in fatal error helper (Sebastian Falbesoner)
9ddf7e03a3 move ThreadImport ABC error to use AbortNode (furszy)

Pull request description:

  It seems odd to return `EXIT_SUCCESS` when the node aborted execution due a fatal internal error
  or any post-init problem that triggers an unrequested shutdown.

  e.g. blocks or coins db I/O errors, disconnect block failure, failure during thread import (external
  blocks loading process error), among others.

ACKs for top commit:
  TheCharlatan:
    ACK 61c569ab60
  ryanofsky:
    Code review ACK 61c569ab60
  pinheadmz:
    ACK 61c569ab60
  theStack:
    Code-review ACK 61c569ab60

Tree-SHA512: 18a59c3acc1c6d12cbc74a20a401e89659740c6477fccb59070c9f97922dfe588468e9e5eef56c5f395762187c34179a5e3954aa5b844787fa13da2e666c63d3

src/interfaces/node.h

@@ -80,6 +80,9 @@ public:
     //! Get warnings.
     virtual bilingual_str getWarnings() = 0;
 
+    //! Get exit status.
+    virtual int getExitStatus() = 0;
+
     // Get log flags.
     virtual uint32_t getLogCategories() = 0;