[foreign-jextract] RFR: 8259473: jextract generated code should throw exception for unfound native symbols from calls, variable access, set immediately

[Athijegannathan Sundararajan]

NPE with proper error message is thrown immediately (on call, variable access/set)

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Commit messages:
 - 8259473: jextract generated code should throw exception for unfound native symbols from calls, variable access, set immediately

Changes: https://git.openjdk.java.net/panama-foreign/pull/432/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=432&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259473
  Stats: 189 lines in 4 files changed: 181 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/432.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/432/head:pull/432

PR: https://git.openjdk.java.net/panama-foreign/pull/432