bpd: error instead of crashing on extra argument

If an MPC client is expecting a command to take an argument that bpd
isn't expecting (e.g. because of a difference in protocol versions) then
bpd currently crashes completely. Instead, do what the real MPD does and
return an error message over the protocol.

beetsplug/bpd/__init__.py

@@ -674,7 +674,8 @@ class Command(object):
         # Attempt to get correct command function.
         func_name = 'cmd_' + self.name
         if not hasattr(conn.server, func_name):
-            raise BPDError(ERROR_UNKNOWN, u'unknown command', self.name)
+            raise BPDError(ERROR_UNKNOWN,
+                           u'unknown command "{}"'.format(self.name))
         func = getattr(conn.server, func_name)
 
         # Ensure we have permission for this command.
@@ -690,6 +691,13 @@ class Command(object):
                 for data in results:
                     yield conn.send(data)
 
+        except TypeError:
+            # The client provided too many arguments.
+            raise BPDError(ERROR_ARG,
+                           u'wrong number of arguments for "{}"'
+                           .format(self.name),
+                           self.name)
+
         except BPDError as e:
             # An exposed error. Set the command name and then let
             # the Connection handle it.

test/test_player.py

@@ -347,6 +347,11 @@ class BPDTest(BPDTestHelper):
             response = client.send_command('notacommand')
         self._assert_failed(response, bpd.ERROR_UNKNOWN)
 
+    def test_unexpected_argument(self):
+        with self.run_bpd() as client:
+            response = client.send_command('clearerror', 'extra argument')
+        self._assert_failed(response, bpd.ERROR_ARG)
+
 
 class BPDQueryTest(BPDTestHelper):
     test_implements_query = implements({