Attempt to report problems with the Berksfile more accurately

I ran into a problem where a cookbook was using a metadata.rb which read in a README.md e.g.

metadata.rb:
```ruby
long_description  IO.read(File.join(File.dirname(__FILE__), 'README.md'))
```

If the README.md is missing then one gets this error message: `No Berksfile or Berksfile.lock found at: <path>/Berksfile`

The exception contained the error message: `No such file or directory - <path>/cookbooks/role/README.md`

This patch will be less confusing when there is a problem with a `metadata.rb file` and will expose the real problem instead of blaming the `Berksfile`.