patch 8.2.3082: a channel command "echoerr" does not show anything

Problem:    A channel command "echoerr" does not show anything.
Solution:   Do not use silent errors when using an "echoerr" command.
            (closes #8494)
diff --git a/src/testdir/test_channel.py b/src/testdir/test_channel.py
index 8dba3ba..9684bb9 100644
--- a/src/testdir/test_channel.py
+++ b/src/testdir/test_channel.py
@@ -109,6 +109,11 @@
                         print("sending: {0}".format(cmd))
                         self.request.sendall(cmd.encode('utf-8'))
                         response = "ok"
+                    elif decoded[1] == 'echoerr':
+                        cmd = '["ex","echoerr \\\"this is an error\\\""]'
+                        print("sending: {0}".format(cmd))
+                        self.request.sendall(cmd.encode('utf-8'))
+                        response = "ok"
                     elif decoded[1] == 'bad command':
                         cmd = '["ex","foo bar"]'
                         print("sending: {0}".format(cmd))