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Current implementation doesn't always make it obvious what the exact
problem with the URL is, so this makes it clearer by consistently quoting
the invalid URL, as is the norm in other parsing implementations, eg.:
strconv.Atoi(" 123") returns an error: parsing " 123": invalid syntax

Updates #29261

Change-Id: Icc6bff8b4a4584677c0f769992823e6e1e0d397d
GitHub-Last-Rev: 648b9d93fe149ec90f3aeca73019158a344de03e
GitHub-Pull-Request: golang/go#29384
Reviewed-on: https://go-review.googlesource.com/c/go/+/185117

Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>