Better error msg

Author: YingboMa

src/systems/connectors.jl

@@ -274,7 +274,7 @@ function expand_connections(sys::AbstractSystem; debug=false, tol=1e-10)
         len = length(outers) + length(inners)
         allconnectors = Iterators.flatten((outers, inners))
         dups = find_duplicates(nameof(c) for c in allconnectors)
-        length(dups) == 0 || error("$(Connection(sys)) has duplicated connections: $(dups).")
+        length(dups) == 0 || error("Connection([$(join(map(nameof, allconnectors), ", "))]) has duplicated connections: [$(join(collect(dups), ", "))].")
     end
 
     if debug