Make error messages more helpful

When passing the wrong number of args to a command, it will now tell you
what args were passed.

This is useful in cases where it is unclear what the arguments are. For
example, we were invoking the binary from a script which was not
properly escaping the arguments that it was forwarding. This caused the
invocation `--option="1 2"` to be passed as `--option=1 2` and `2` became
an argument to the command, which took about half an hour to figure out
(plus, the hour my QA spent trying to figure it out herself before I
could look at it with her). If it had told me that it was receiving the
argument `2`, it would have been pretty clear that it was an escaping
issue.

I removed the arity from the message because in the case of optional
arguments, it would output a negative number, which is confusing for
non-rubyists.

I didn't include the signature, because 1.8.7 was listed in the
.travis.yml, and it does not have the ability to reflect on parameters.
Plus, again, if the end-user is not a rubyist, then that kind of error
message would probably be too technical. So instead, just print the
description.
Remove unused method bodies from fixture
Error message wording

Implements changes suggested by sferik in https://github.com/wycats/thor/pull/312

In the error messages for handling incorrect number of arguments

* Remove branch from if statement that will never be hit.
* Put error message on two lines
* Use "arguments" instead of "args"
Merge pull request #312 from JoshCheek/master

Make error messages more helpful