RFR: JDK-8297000 [jib] Add more friendly warning for proxy issues [v3]

[Ludvig Janiuk]

> When jib tries to download itself behind a proxy, sometimes it can get a different file than it expected. This can result in cryptic errors that are hard to troubleshoot. Let's handle this a little more gracefully by detecting it in the bootstrapper and printing an error.

Ludvig Janiuk has updated the pull request incrementally with one additional commit since the last revision:

  ws

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11159/files
  - new: https://git.openjdk.org/jdk/pull/11159/files/78e846ec..fcd186bb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11159&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11159&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11159.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11159/head:pull/11159

PR: https://git.openjdk.org/jdk/pull/11159