[1.5.x] Fixed #19257 -- Don't swallow command's KeyError in call_command

Thanks Giovanni Bajo for the report.
Backport of 9a09558 from master.

django/core/management/__init__.py

@@ -136,14 +136,15 @@ def call_command(name, *args, **options):
     # Load the command object.
     try:
         app_name = get_commands()[name]
-        if isinstance(app_name, BaseCommand):
-            # If the command is already loaded, use it directly.
-            klass = app_name
-        else:
-            klass = load_command_class(app_name, name)
     except KeyError:
         raise CommandError("Unknown command: %r" % name)
 
+    if isinstance(app_name, BaseCommand):
+        # If the command is already loaded, use it directly.
+        klass = app_name
+    else:
+        klass = load_command_class(app_name, name)
+
     # Grab out a list of defaults from the options. optparse does this for us
     # when the script runs from the command line, but since call_command can
     # be called programatically, we need to simulate the loading and handling