Error on broken cocoapods installation

We currently check if there is some program called `pod` on the user machine and error out if there is not.

If a cocoapods installation is corrupted, we fail at `PodSpec` conversion time producing an error message that would lead the user to believe the podspec is invalid. ```_exec failed 4 fatal error: Invalid JSON Podspec:```

Simply check `pod --version` or similar to get a better sense of if the program is correctly installed, and print a nice error if it isn't.