New cop: system vs sh

[pocke]

In Rake task, I prefer sh to system to execute external commands.
Because sh raises an error when the command exits with non-zero. It is safer.

task :foo do
  system 'false' # It does not raise any errors
  sh 'false' # It raises an error.
end

Note: I think system in not void context is ok. So the cop should not add any offense for the following code.

task :foo do
  # It does not raise, but handled.
  if system 'cmd'
    # ...
  end
end