Suppress 'Maybe IRB bug!' message in extract_ruby_args

[tompng]

Improve error message of #958

irb(main):001> measure = 1
undefined method `extract_ruby_args=' for module IRB::Command (NoMethodError)
IRB::Command.extract_ruby_args = 1
            ^^^^^^^^^^^^^^^^^^^^
Maybe IRB bug!

This is not a bug. "= 1" is an invalid argument for measure command.

irb(main):002> measure a + b
undefined local variable or method `a' for main (NameError)
IRB::Command.extract_ruby_args a + b
                               ^
Maybe IRB bug!

This is also not a bug.

This pull request improves the error message with less effort.
Note that IRB::Command::RubyArgsExtractor is a backward-compatibility layer of command arguments and we are going to reduce using it.

[tompng]

#961 will solve most of the problem.