log err instead of throwing when no shmem regions found

include/storage/shared_monitor.hpp

@@ -117,6 +117,17 @@ template <typename Data> struct SharedMonitor
 #endif
 
     static void remove() { bi::shared_memory_object::remove(Data::name); }
+    static bool exists() {
+        try
+        {
+            bi::shared_memory_object shmem_open = bi::shared_memory_object(bi::open_only, Data::name, bi::read_only);
+        }
+        catch (const bi::interprocess_exception &exception)
+        {
+            return false;
+        }
+        return true;
+    }
 
   private:
 #if USE_BOOST_INTERPROCESS_CONDITION

src/tools/store.cpp

@@ -27,7 +27,11 @@ void deleteRegion(const storage::SharedRegionRegister::ShmKey key)
 
 void listRegions()
 {
-
+    if (!storage::SharedMonitor<storage::SharedRegionRegister>::exists())
+    {
+        osrm::util::Log() << "No shared memory regions found. Try running osrm-datastore";
+        return;
+    }
     storage::SharedMonitor<storage::SharedRegionRegister> monitor;
     std::vector<std::string> names;
     const auto &shared_register = monitor.data();
@@ -105,8 +109,7 @@ bool generateDataStoreOptions(const int argc,
          boost::program_options::value<bool>(&list_datasets)
              ->default_value(false)
              ->implicit_value(true),
-         "Name of the dataset to load into memory. This allows having multiple datasets in memory "
-         "at the same time.") //
+         "List all OSRM datasets currently in memory") //
         ("only-metric",
          boost::program_options::value<bool>(&only_metric)
              ->default_value(false)