Refactored get_metadata_section to return a Result<T,~str>, added error messages. Closes #6615.
auto merge of #12758 : rgawdzik/rust/master, r=alexcrichton

Refactored get_metadata_section to return a Result<MetadataBlob,~str> instead of a Option<MetadataBlob>. This provides more clarity to the user through the debug output when using --ls.

This is kind of a continuation of my original closed pull request 2 months ago (#11544), but I think the time-span constitutes a new pull request.