Route unknown error to parent exception's handler

src/kemal/exception_handler.cr

@@ -13,20 +13,38 @@ module Kemal
     rescue ex : Exception
       # Use error handler defined for the current exception if it exists
       return call_exception_with_exception(context, ex, 500) if Kemal.config.error_handlers.has_key?(ex.class)
+
+      # Use error handler for an ancestor of the current exception if it exists
+      Kemal.config.error_handlers.each_key do |key|
+        if key.is_a? Exception.class && ex.class <= key
+          return call_exception_with_exception(context, ex, 500, override_handler_used: key)
+        end
+      end
+
       log("Exception: #{ex.inspect_with_backtrace}")
       # Else use generic 500 handler if defined
       return call_exception_with_status_code(context, ex, 500) if Kemal.config.error_handlers.has_key?(500)
       verbosity = Kemal.config.env == "production" ? false : true
       render_500(context, ex, verbosity)
     end
 
-    private def call_exception_with_exception(context : HTTP::Server::Context, exception : Exception, status_code : Int32 = 500)
+    # Calls the defined error handler for the given exception if it exists
+    #
+    # By default it tries to use the handler that is defined for the given exception. However, another
+    # handler can be used via the `override_handler_used` parameter.
+    private def call_exception_with_exception(context : HTTP::Server::Context, exception : Exception, status_code : Int32 = 500, override_handler_used : (Exception.class)? = nil)
       return if context.response.closed?
 
-      if !Kemal.config.error_handlers.empty? && Kemal.config.error_handlers.has_key?(exception.class)
+      if !override_handler_used
+        handler_to_use = exception.class
+      else
+        handler_to_use = override_handler_used
+      end
+
+      if !Kemal.config.error_handlers.empty? && Kemal.config.error_handlers.has_key?(handler_to_use)
         context.response.content_type = "text/html" unless context.response.headers.has_key?("Content-Type")
         context.response.status_code = status_code
-        context.response.print Kemal.config.error_handlers[exception.class].call(context, exception)
+        context.response.print Kemal.config.error_handlers[handler_to_use].call(context, exception)
         context
       end
     end