RFR: 8220355: Improve assertion texts and exception messages in eventHandlerVMInit

[Baesken, Matthias]

Hello, please review the following change .

It enhances  the error  messages in libinstrument in case initialization fails .

( I found  the enhanced error messages helpful when   analyzing   jtreg failures  on one of our platforms in the  java/lang/instrument/PremainClass/PremainClassTest.java   )



Bug/webrev :

https://bugs.openjdk.java.net/browse/JDK-8220355

http://cr.openjdk.java.net/~mbaesken/webrevs/8220355.0/


Thanks, Matthias
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