RFR: JDK-8290126: Add a check in JavadocTester for "javadoc should not crash"

[Jonathan Gibbons]

Please review a simple update to `JavadocTester` to provide a default-on check that no stack traces were written to the STDERR stream.

There are also some minor cleanup edits suggested by the IDE.

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Commit messages:
 - JDK-8290126: Add a check in JavadocTester for "javadoc should not crash"

Changes: https://git.openjdk.org/jdk/pull/9458/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9458&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290126
  Stats: 25 lines in 1 file changed: 14 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/9458.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9458/head:pull/9458

PR: https://git.openjdk.org/jdk/pull/9458