RFR: 8297311: Improve exception message thrown by java.net.HostPortrange::toLowerCase(String s)

[Jaikiran Pai]

Can I please get a review of this change which proposes to enhance an exception message in `java.net.HostPortrange::toLowerCase`, as requested in https://bugs.openjdk.org/browse/JDK-8297311?

The commit here includes (only) the character, which was found to be invalid, in the exception message. The character will be printed out in `\uXXXX` format to take into account any unprintable characters.

A new test has been introduced to verify that the introduction of this change doesn't introduce any regression and does indeed include the expected exception message.

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Commit messages:
 - 8297311: Improve exception message thrown by java.net.HostPortrange::toLowerCase(String s)

Changes: https://git.openjdk.org/jdk/pull/11407/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11407&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297311
  Stats: 59 lines in 2 files changed: 58 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11407.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11407/head:pull/11407

PR: https://git.openjdk.org/jdk/pull/11407