RFR: JDK-8283280: Improve exception messages with -Dsun.misc.URLClassPath.disableJarChecking=false

[Liam Miller-Cushon]

This change improves an exception message to include a path, e.g. `Invalid Jar file: some.jar` instead of just `Invalid Jar file`.

The exception is currently [always being caught and ignored](https://github.com/openjdk/jdk/blob/3e393047e12147a81e2899784b943923fc34da8e/src/java.base/share/classes/jdk/internal/loader/URLClassPath.java#L459-L461) so this visible to users of the API, it's just helpful for debugging.

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Commit messages:
 - JDK-8283280: Improve exception messages with -Dsun.misc.URLClassPath.disableJarChecking=false

Changes: https://git.openjdk.java.net/jdk/pull/7848/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7848&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283280
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7848.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7848/head:pull/7848

PR: https://git.openjdk.java.net/jdk/pull/7848