daemon: ~PathLocks(): Handle exceptions.

Otherwise, since the call to write a "d" character to the lock file
can fail with ENOSPC, we can get an unhandled exception resulting in a
call to terminate().

nix/libstore/pathlocks.cc

@@ -162,7 +162,11 @@ bool PathLocks::lockPaths(const PathSet & _paths,
 
 PathLocks::~PathLocks()
 {
+    try {
         unlock();
+    } catch (...) {
+        ignoreException();
+    }
 }