RFR: 8324126: Error message for mistyping -XX:+Unlock...Options is not helpful [v3]

[Sonia Zaldana Calles]

> If we run the VM option to unlock experimental vm options without the preceding ‘+’, the error message is not very helpful: 
> 
> ```Error: VM option 'UnlockExperimentalVMOptions' is experimental and must be enabled via -XX:+UnlockExperimentalVMOptions.```
> 
> With the proposed change, we should see a more descriptive error message, namely the lack of a +/- sign. 
> 
> ```Missing +/- setting for VM option 'UnlockExperimentalVMOptions'```
> 
> Testing: runtime/commandLine regression tests pass. Also added a regression test to verify expected behaviour.

Sonia Zaldana Calles has updated the pull request incrementally with one additional commit since the last revision:

  restoring end of file

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17575/files
  - new: https://git.openjdk.org/jdk/pull/17575/files/afb70cb8..557f5cc6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17575&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17575&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17575.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17575/head:pull/17575

PR: https://git.openjdk.org/jdk/pull/17575