RFR: 8259343: [macOS] Update JNI error handling in Cocoa code.

[Phil Race]

On Wed, 6 Jan 2021 21:14:06 GMT, Phil Race <prr at openjdk.org> wrote:

> Proposed updates to JNI error handling.

src/java.desktop/macosx/native/libosxapp/JNIUtilities.h line 46:

> 44:               if ((*env)->ExceptionOccurred(env) != NULL) { \
> 45:                   (*env)->ExceptionDescribe(env); \
> 46:               } \

So the update here is that if we are not on the appkit thread, make sure a java exception is thrown.
If we are on the appkit thread, clear any java exception since it isn't going anywhere but do it using
describe which prints it.

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PR: https://git.openjdk.java.net/jdk/pull/1967