TGenEpEmv1: Return bool to indicate init success

Currently TGenEpEmv1 fails when the initialization
fails (sampling of X-section).
This is sub-optimal since such abort will bring down
complete MC jobs the GRID.

This commit allows the caller to react to such error
condition. The caller may decide what to do (give it another go or to
exit).

Relates to https://alice.its.cern.ch/jira/browse/O2-3825

TEPEMGEN/TGenEpEmv1.cxx

@@ -109,7 +109,7 @@ TGenEpEmv1::~TGenEpEmv1()
 }
 
 //____________________________________________________________
-void TGenEpEmv1::Init()
+bool TGenEpEmv1::Init()
 {
   // Initialisation:
   // 1) define a generator
@@ -124,9 +124,10 @@ void TGenEpEmv1::Init()
   double err = 0;
   fXSection = CalcXSection(fXSectionEps,fMinXSTest,fMaxXSTest,err);
   if (fXSection<=0 || err/fXSection>fXSectionEps) {
-    abort();
+    return false;
   }
   fXSectionEps = err/fXSection;
+  return true;
 }
 
 //____________________________________________________________

TEPEMGEN/TGenEpEmv1.h

@@ -21,10 +21,10 @@ class TGenEpEmv1 : public TEpEmGen {
 
  public:
   TGenEpEmv1();
-  virtual ~TGenEpEmv1();
+  ~TGenEpEmv1() override;
 
-  virtual void GenerateEvent();
-  virtual void Init();
+  void GenerateEvent() override; // interface of TGenerator
+  bool Init(); // init function; returns true if successful; false otherwise
   void SetDebug(Int_t debug) {fDebug=debug;}
   void SetYRange(Double_t min, Double_t max) {fYMin = min; fYMax = max;};
   void SetPtRange(Double_t min, Double_t max) {fPtMin = min; fPtMax = max;};