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

[Chris Plummer]

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.

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Commit messages:
 - Cleanup error handling

Changes: https://git.openjdk.org/jdk/pull/10149/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10149&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293325
  Stats: 15 lines in 1 file changed: 8 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/10149.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10149/head:pull/10149

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