Do not execute anything if command line is invalid, and make it the default behavior

[rayluo]

I know a similar issue has been mentioned in #222 (which should have been labelled as Duplicated rather than a misleadingly Closed) , #226, and its root cause was well-explained in #168.

It is still worth to point out that, this issue can be much more easier to hit, thus much more severe than those somewhat contrived scenarios described in #222 and #168 (which was triggered by mismatching default parameters and/or typo).

# This is part of an CRUD app
def add(name):
    print("Record %s has been added into database" % name)

if __name__ == "__main__":
    fire.Fire()

My end user (or me) could easily run it with $python my_app.py add John Doe (forgot to add quotation marks) and then a record "John" would have been added into database while the end user saw a command-line error. At that point, it is not just about issuing the right command, the end user need to clean up the messed-up database.

Although a decorator workaround was mentioned in #168, do you think it is better to do it the other way around, i.e. make such "strict mode" (#226) the default behavior, and let developer to chain commands if they really want to? I bet not many developer know and anticipate the command chaining behavior.