ipc: Handle bitcoin-wallet disconnections

This fixes an error reported by Antoine Poinsot <darosior@protonmail.com> in
https://github.com/bitcoin-core/libmultiprocess/issues/123 that does not happen
in master, but does happen with https://github.com/bitcoin/bitcoin/pull/10102
applied, where if the child bitcoin-wallet process is killed (either by an
external signal or by Ctrl-C as reported in the issue) the bitcoin-node process
will not shutdown cleanly after that because chain client flush()
calls will fail.

This change fixes the problem by handling ipc::Exception errors thrown during
the flush() calls, and it relies on the fixes to disconnect detection
implemented in https://github.com/bitcoin-core/libmultiprocess/pull/160 to work
effectively.

src/test/util/setup_common.cpp

@@ -214,7 +214,10 @@ BasicTestingSetup::~BasicTestingSetup()
     } else {
         fs::remove_all(m_path_root);
     }
+    // Clear all arguments except for -datadir, which GUI tests currently rely
+    // on to be set even after the testing setup is destroyed.
     gArgs.ClearArgs();
+    gArgs.ForceSetArg("-datadir", fs::PathToString(m_path_root));
 }
 
 ChainTestingSetup::ChainTestingSetup(const ChainType chainType, TestOpts opts)