[jdk11u-dev] RFR: 8297000: [jib] Add more friendly warning for proxy issues

[Goetz Lindenmaier]

I backport this for parity with 11.0.20-oracle.

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Commit messages:
 - Backport 0d93ab9dff04409469faa26d5cdb29c4d6488537

Changes: https://git.openjdk.org/jdk11u-dev/pull/1832/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1832&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297000
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1832.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/1832/head:pull/1832

PR: https://git.openjdk.org/jdk11u-dev/pull/1832