Return EXIT_FAILURE on post-init fatal errors

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.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

test/functional/feature_abortnode.py

@@ -40,7 +40,7 @@ class AbortNodeTest(BitcoinTestFramework):
 
             # Check that node0 aborted
             self.log.info("Waiting for crash")
-            self.nodes[0].wait_until_stopped(timeout=5)
+            self.nodes[0].wait_until_stopped(timeout=5, expect_error=True)
         self.log.info("Node crashed - now verifying restart fails")
         self.nodes[0].assert_start_raises_init_error()