cli: fix crash when dfu action is omitted

Before:

    $ pybricksdev dfu
    Traceback (most recent call last):
    File "/home/david/work/pybricks/pybricksdev/.venv/bin/pybricksdev", line 5, in <module>
        main()
    File "/home/david/work/pybricks/pybricksdev/pybricksdev/cli/__init__.py", line 347, in main
        asyncio.run(subparsers.choices[args.tool].tool.run(args))
    File "/home/david/work/pybricks/pybricksdev/pybricksdev/cli/__init__.py", line 258, in run
        return self.subparsers.choices[args.action].tool.run(args)
    KeyError: None

After:

    $ pybricksdev dfu
    usage: pybricksdev dfu [-h] <action> ...
    pybricksdev dfu: error: Missing name of action: backup|restore

Author: dlech

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `pybricksdev.ble.lwp3.bytecodes` module.
 - `pybricksdev.ble.lwp3.messages` module.
 - `pybricksdev lwp3 repl` command line tool.
+## Fixed
+- Crash when running `pybricksdev dfu` without args on command line.
 
 ## [1.0.0-alpha.8] - 2021-05-18
 ## Added

pybricksdev/cli/__init__.py

@@ -242,19 +242,24 @@ async def run(self, args: argparse.Namespace):
 
 class DFU(Tool):
     def add_parser(self, subparsers: argparse._SubParsersAction):
-        parser = subparsers.add_parser(
+        self.parser = subparsers.add_parser(
             "dfu",
             help="use DFU to backup or restore firmware",
         )
-        parser.tool = self
-        self.subparsers = parser.add_subparsers(
+        self.parser.tool = self
+        self.subparsers = self.parser.add_subparsers(
             metavar="<action>", dest="action", help="the action to perform"
         )
 
         for tool in DFUBackup(), DFURestore():
             tool.add_parser(self.subparsers)
 
     def run(self, args: argparse.Namespace):
+        if args.action not in self.subparsers.choices:
+            self.parser.error(
+                f'Missing name of action: {"|".join(self.subparsers.choices.keys())}'
+            )
+
         return self.subparsers.choices[args.action].tool.run(args)