Improve error message when library metadata request fails

[jimmylewis]

This issue is dedicated to one of the follow-up items in this comment: #370 (comment)

Functional impact

Error is imprecise about the cause of failure and may lead to user confusion

Minimal repro steps

Happens randomly, no reliable repro
When downloading a library for the first time, if one of the metadata requests fails we get a generic "Could not resolve the library [email protected]".

Expected result

Better indicate the cause of failure or the portion which failed, some ideas:

• Failed to retrieve file metadata for [email protected]
• Failed to retrieve library metadata for [email protected]. Received HTTP response 418.
• Unrecognized library name "foo"
• Unrecognized library version for "[email protected]"

Actual result

All issues resolving the library and its metadata get a "Could not resolve the library [email protected]" message.