RFR: JDK-8283668: Update IllegalFormatException to use sealed classes

[Joe Darcy]

Working down the list of candidates to be sealed, this time IllegalFormatException.

Please also review the companion CSR: https://bugs.openjdk.java.net/browse/JDK-8283669

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Commit messages:
 - JDK-8283668: Update IllegalFormatException to use sealed classes

Changes: https://git.openjdk.java.net/jdk/pull/7950/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7950&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283668
  Stats: 38 lines in 13 files changed: 12 ins; 0 del; 26 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7950.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7950/head:pull/7950

PR: https://git.openjdk.java.net/jdk/pull/7950