Check presence of hit files before adding to TChain

This reduces bogus error messages about missing hit files
(in particular for ZDC) and improves the handling of
setting up the hits for digitization.

(cherry picked from commit 43223a4)

Author: sawenzel

DataFormats/simulation/include/SimulationDataFormat/DigitizationContext.h

@@ -209,6 +209,9 @@ inline void DigitizationContext::retrieveHits(std::vector<TChain*> const& chains
                                               int entryID,
                                               std::vector<T>* hits) const
 {
+  if (chains.size() <= sourceID) {
+    return;
+  }
   auto br = chains[sourceID]->GetBranch(brname);
   if (!br) {
     LOG(error) << "No branch found with name " << brname;

DataFormats/simulation/src/DigitizationContext.cxx

@@ -99,6 +99,14 @@ bool DigitizationContext::initSimChains(o2::detectors::DetID detid, std::vector<
     return false;
   }
 
+  // check that all files are present, otherwise quit
+  for (int source = 0; source < mSimPrefixes.size(); ++source) {
+    if (!std::filesystem::exists(o2::base::DetectorNameConf::getHitsFileName(detid, mSimPrefixes[source].data()))) {
+      LOG(info) << "Not hit file present for " << detid.getName() << " (exiting SimChain setup)";
+      return false;
+    }
+  }
+
   simchains.emplace_back(new TChain("o2sim"));
   // add the main (background) file
   simchains.back()->AddFile(o2::base::DetectorNameConf::getHitsFileName(detid, mSimPrefixes[0].data()).c_str());