Auto merge of #323 - Enet4:imp/no-unwrap-in-doctests, r=SimonSapin

Replace unwrap() in doctests with `?`

Fixes #312.
 - All results yielding `ParseError` in the doctests should now be handled with the ? operator.
 - Some other specific results are ~unwrapped with `expect()`~ mapped to a placeholder string message, such as in URLs that are not _cannot-be-a-base_. Users are then expected to adjust this mapping depending on their use case. Once #299 is resolved, we'll be able to propagate these normally.

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Author: bors-servo