Fix help in prompt-mode

This is done with catching TypeError and calling
topydo.cli.CLIApplicationsBase.usage.

topydo/cli/Prompt.py

@@ -18,7 +18,7 @@
 
 import sys
 
-from topydo.cli.CLIApplicationBase import CLIApplicationBase, error
+from topydo.cli.CLIApplicationBase import CLIApplicationBase, error, usage
 from topydo.cli.TopydoCompleter import TopydoCompleter
 from prompt_toolkit.shortcuts import get_input
 from prompt_toolkit.history import History
@@ -66,6 +66,7 @@ class PromptApplication(CLIApplicationBase):
 
             (subcommand, args) = get_subcommand(user_input)
 
+            try:
                 if self._execute(subcommand, args) != False:
                     if self.todolist.is_dirty():
                         self._archive()
@@ -74,6 +75,8 @@ class PromptApplication(CLIApplicationBase):
                             self._execute(SortCommand, [])
 
                         self.todofile.write(str(self.todolist))
+            except TypeError:
+                usage()
 
 def main():
     """ Main entry point of the CLI. """