Update bad config error.

modules/exploits/multi/gdb/gdb_server_exec.rb

@@ -35,8 +35,12 @@ class Metasploit3 < Msf::Exploit::Remote
     print_status "Stepping program to find PC..."
     gdb_pc, gdb_arch = process_info.values_at :pc, :arch
 
-    unless arch.include? gdb_arch
-      fail_with('The payload architecture is incorrect.')
+    unless payload.arch.include? gdb_arch
+      fail_with(
+        Msf::Exploit::Failure::BadConfig,
+        "The payload architecture is incorrect: "+
+        "the payload is #{payload.arch.first}, but #{gdb_arch} was detected from gdb."
+      )
     end
 
     print_status "Writing payload at #{gdb_pc}..."