Don't report errors for SystemExit. Closes #2442.
[Command] Remove Pod::Command::Help

Closes #2156.
[Command] Use a case statement in Pod::Command#report_error

Addresses https://github.com/CocoaPods/CocoaPods/commit/0c80af37f1fec0009ad63277309afa47ad22595f#commitcomment-7718723.