Vague error message for 0.0.0-pre

This looks like an edge case but I struggled to diagnose it for a few hours.

My podspec looks like this:

```
Pod::Spec.new do |s|
s.name = 'MyLib'
s.version = '0.0.0-alpha.0'
...
```

When I run `pod spec lint MyLib.podspec`, I get the following error, produced by Molinillo:

```
- ERROR | [iOS] unknown: Encountered an unknown error (CocoaPods could not find compatible versions for pod "MyLib":
In Podfile:
MyLib (from `/Users/bnickel/Documents/GitHub/my-lib/MyLib.podspec`)
```

After some experimentation, I discovered that any pre-release label on 0.0.0 triggers this error. Validation continues if I use `0.0.0` or `0.0.1-alpha.0`. This could either be resolved by adding support for 0.0.0 pre-releases or improving the message that gets logged in this case.

Reproduce the issue with the shown MyLib.podspec and `pod spec lint MyLib.podspec`, then trace the Molinillo dependency-resolution path for a `0.0.0` pre-release. The issue leaves two possible outcomes open: support the version or improve the logged error; completion requires choosing and validating one of them.

Written by the indexing model from the issue text.