RFR: 8357675: Amend headless message

[Alexey Ivanov]

Currently, the default headless message ends with a comma instead of full stop and has an additional line break.


Exception in thread "AWT-EventQueue-0" java.awt.HeadlessException:
No X11 DISPLAY variable was set,
or no headful library support was found,
but this program performed an operation which requires it,

        at java.desktop/java.awt.GraphicsEnvironment.checkHeadless(GraphicsEnvironment.java:158)
        at java.desktop/java.awt.Window.<init>(Window.java:518)
        at java.desktop/java.awt.Frame.<init>(Frame.java:428)
        at java.desktop/javax.swing.JFrame.<init>(JFrame.java:224) 


**Fix:**

Amend the message so that it ends with a full stop and remove the line break.


Exception in thread "AWT-EventQueue-0" java.awt.HeadlessException:
No X11 DISPLAY variable was set,
or no headful library support was found,
but this program performed an operation which requires it.
        at java.desktop/java.awt.GraphicsEnvironment.checkHeadless(GraphicsEnvironment.java:158)
        at java.desktop/java.awt.Window.<init>(Window.java:518)
        at java.desktop/java.awt.Frame.<init>(Frame.java:428)
        at java.desktop/javax.swing.JFrame.<init>(JFrame.java:224)


**Testing:**

Build a headless-only JDK (pass `--enable-headless-only` to the `configure` script) and create a `JFrame`.

It's impossible to create an automated test for this change, and I don't think a test is required, it's a cosmetic change.

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Commit messages:
 - 8357675: Amend headless message

Changes: https://git.openjdk.org/jdk/pull/25453/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25453&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357675
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25453.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25453/head:pull/25453

PR: https://git.openjdk.org/jdk/pull/25453