refactor: index: use `AbortNode` in fatal error helper

Deduplicates code in the `FatalError` template function by using
`AbortNode` which does the exact same thing if called without any user
message (i.e. without second parameter specified). The template is still
kept for ease-of-use w.r.t. not having to call `tfm::format(...)` at the
call-side each time, and also to keep the diff minimal.

src/index/base.cpp

@@ -33,11 +33,7 @@ constexpr auto SYNC_LOCATOR_WRITE_INTERVAL{30s};
 template <typename... Args>
 static void FatalError(const char* fmt, const Args&... args)
 {
-    std::string strMessage = tfm::format(fmt, args...);
+    AbortNode(tfm::format(fmt, args...));
-    SetMiscWarning(Untranslated(strMessage));
-    LogPrintf("*** %s\n", strMessage);
-    InitError(_("A fatal internal error occurred, see debug.log for details"));
-    StartShutdown();
 }
 
 CBlockLocator GetLocator(interfaces::Chain& chain, const uint256& block_hash)