Ref Tracking: Improve CSR Error Message (#1250)

Reading the user question in
  #44 (comment)
I have the impression my original error message is
too brief and not helpful enough. Trying to be more
verbose now, so users know what to look for/change.

Author: ax3l

src/tracking/reference.cpp

@@ -78,7 +78,11 @@ namespace impactx
         pp_algo.query("csr", csr);
         if (csr)
         {
-            throw std::runtime_error("CSR effects cannot be modeled for single particle tracking.");
+            throw std::runtime_error(
+                "Coherent Synchrotron Radiation (CSR) cannot be "
+                "modeled for single particle tracking. "
+                "Please set the CSR option to false."
+            );
         }
 
         // periods through the lattice