RFR: 1838: better error message when download fails [v3]

[Zhao Song]

> Sometimes, users may want to run jcheck locally before creating PRs. However, Oracle employees who are connected to the Oracle VPN and have not set the https_proxy environment variable may encounter a "connect exception" error. 
> In this patch, we will provide users with more information in the case of a "connect exception" error while running jcheck.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  fix a problem

-------------

Changes:
  - all: https://git.openjdk.org/skara/pull/1486/files
  - new: https://git.openjdk.org/skara/pull/1486/files/8a41dc23..e43519ec

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1486&range=02
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1486&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1486.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1486/head:pull/1486

PR: https://git.openjdk.org/skara/pull/1486