Add more explanation for unresolvable git refs

* Clarify language.
* Show the command that we ran to resolve the branch/tag/whatever
* Add a hint about v1.2.3 vs. 1.2.3
* Use the ErrorReporter to show other relevant info.

Output looks like:

```
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 
Command `git ls-remote origin 3.0.0.beta7*' did not find a commit for reference `3.0.0.beta7'.
The tag or branch you're looking for doesn't exist on the remote repo.
If your project uses version tags like v1.2.3, include the 'v' in your
software's version.
Fetcher params:
  repo URI:       git://github.com/berkshelf/berkshelf
  local location: /var/cache/omnibus/src/berkshelf
Exception:
  Omnibus::UnresolvableGitReference: Could not resolve `3.0.0.beta7' to a SHA.
```