CocoaPods/CocoaPods — pull request #403 by siuying (2012-07-15 03:10:56+00:00)
Title: raise informative error message when user supplied invalid platform

A small patch to add informative error message for error like issue #401.

For following Podfile:

```ruby
platform :iOS

pod 'CoconutKit'
```

Instead of crash on ```pod install`` it now throw:

```ruby
> pod install
[!] Unsupported platform: Platform must be one of [:ios, :osx]
```