[PATCH 0 of 2] detect arguments error for wrapped commands

[Dmitriy Taychenachev]

When command's wrapping handler is being called, hg can't properly
detect arguments error (like insufficient arguments count for import
command, reproducable with wrapper from mq) and throws exception to
user. With this patch hg will try to detect arguments error in very
first wrapping handler (otherwise we can't guarantee that error was
caused by user input and not previous handler's bug) so every handler
should be careful when arguments list when passing to inherited
handler.