[foreign-jextract] RFR: 8261337: jextract error message when directory is passed as header file argument

[Athijegannathan Sundararajan]

directory-as-header is detected upfront before parsing and proper error message is issued.

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Commit messages:
 - 8261337: jextract error message when directory is passed as header file argument

Changes: https://git.openjdk.java.net/panama-foreign/pull/448/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=448&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261337
  Stats: 14 lines in 5 files changed: 13 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/448.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/448/head:pull/448

PR: https://git.openjdk.java.net/panama-foreign/pull/448