(https://github.com/ruby/irb/pull/473)

* Handle file loading commands' argument error gracefully

Currently, if users don't provide an argument to `source`,
`irb_load`, and `irb_require`, IRB raises `ArgumentError` with full
stacktrace. This is confusing because it looks similar to when IRB has
internal issues. The message also isn't helpful on helping users avoid
the error.

So in this commit, I add a new `CommandArgumentError` for commands to
raise explicitly when users' input doesn't satisfy a command's argument
requirement.

* Gracefully handle `fg` command's argument requirement