Adds assertion to catch going back more than is possible

FlowStacks.podspec

@@ -1,7 +1,7 @@
 Pod::Spec.new do |s|
 
   s.name             = 'FlowStacks'
-  s.version          = '0.2.0'
+  s.version          = '0.2.1'
   s.summary          = 'Hoist navigation state into a coordinator in SwiftUI.'
 
   s.description      = <<-DESC

Sources/FlowStacks/RoutableCollection+utilities.swift

@@ -62,6 +62,10 @@ public extension RoutableCollection where Element: RouteProtocol {
   /// Goes back a given number of screens off the stack
   /// - Parameter count: The number of screens to go back. Defaults to 1.
   mutating func goBack(_ count: Int = 1) {
+    assert(
+      self.count - count >= 0,
+      "Can't go back\(count == 1 ? "" : " \(count) screens") - the screen count is \(self.count)"
+    )
     guard self.count - count >= 0 else { return }
     removeLast(count)
   }