RFR: JDK-8279242: Reflection newInstance() error message when constructor has no access modifiers could use improvement

[Joe Darcy]

Making the exception message friendlier to users.

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Commit messages:
 - JDK-8279242: Reflection newInstance() error message when constructor has no access modifiers could use improvement

Changes: https://git.openjdk.java.net/jdk/pull/7208/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7208&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279242
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7208.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7208/head:pull/7208

PR: https://git.openjdk.java.net/jdk/pull/7208