RFR: 8293325: Minor improvements to macos catch_mach_exception_raise() error handling

[Chris Plummer]

On Fri, 2 Sep 2022 18:49:27 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

> While trying to find a fix for [JDK-8288429](https://bugs.openjdk.org/browse/JDK-8288429) (no fix was found), I made some adjustments to the error handling in catch_mach_exception_raise() to make it a bit easier to understand the code, and to provide a more meaningful error message when the soft signal is invalid. This PR is for pushing those changes.

Thanks Dan, Alex, and Serguei!

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PR: https://git.openjdk.org/jdk/pull/10149