RFR: 8286057: Make javac error on a generic enum friendlier

[Aggelos Biboudis]

Improves the error message for generic enumerations.

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Commit messages:
 - 8286057: Make javac error on a generic enum friendlier

Changes: https://git.openjdk.java.net/jdk/pull/8677/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8677&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286057
  Stats: 71 lines in 5 files changed: 71 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8677.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8677/head:pull/8677

PR: https://git.openjdk.java.net/jdk/pull/8677