RFR: 8287397 : Print top-level exception when snippet fails to read file

[Anthony Vanelverdinghe]

When using a snippet as follows (on Windows): `{@snippet file=baz\Baz.java }`, javadoc prints the error:

src\foo\foo\Foo.java:14: error: Error reading file: baz\Baz.java
{@snippet file=baz\Baz.java }
          ^
        (null)


which is unhelpful. So rather than printing the exception's cause (which is `null` in the example above), it seems better to print the top-level exception itself.

PS: I'll need help from someone to create a JBS issue & sponsor this PR

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Commit messages:
 - Print top-level exception

Changes: https://git.openjdk.org/jdk/pull/8826/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=8826&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287397
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/8826.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/8826/head:pull/8826

PR: https://git.openjdk.org/jdk/pull/8826