RFR: 8282881: Print exception message in VM crash with -XX:AbortVMOnException

[Emanuel Peter]

In `Exceptions::debug_check_abort`, we crash the VM if the exception matches with `-XX:AbortVMOnException`. For example `-XX:AbortVMOnException=java.lang.RuntimeEx`.

Currently, in the VM crash description, we only print the exception name (`value_string`), and not its message (`message`). For completeness and consistency, we should also print the exception message.

I tested it with these two exceptions, the first results in `message` being `NULL`:
`throw new RuntimeException();`
`throw new RuntimeException("some message");`

Running tests to make sure nothing else broke.

-------------

Commit messages:
 - 8282881: Print exception message in VM crash with -XX:AbortVMOnException

Changes: https://git.openjdk.java.net/jdk/pull/7762/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7762&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282881
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7762.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7762/head:pull/7762

PR: https://git.openjdk.java.net/jdk/pull/7762