Fix abort when the GC thread gets an exception

src/libstore/gc.cc

@@ -887,6 +887,8 @@ void LocalStore::autoGC(bool sync)
 
         std::thread([promise{std::move(promise)}, this, avail, getAvail]() mutable {
 
+            try {
+
                 /* Wake up any threads waiting for the auto-GC to finish. */
                 Finally wakeup([&]() {
                     auto state(_state.lock());
@@ -906,6 +908,12 @@ void LocalStore::autoGC(bool sync)
 
                 _state.lock()->availAfterGC = getAvail();
 
+            } catch (...) {
+                // FIXME: we could propagate the exception to the
+                // future, but we don't really care.
+                ignoreException();
+            }
+
         }).detach();
     }