RFR: 7903134: Imrpove error reporting in jextract gradle build

[Jorn Vernee]

Improve error reporting around missing LLVM paths in jextract gradle build.

This patch tries to detect missing paths and throw a descriptive error, instead of a more cryptic one.

I've also added the `.idea` folder to the `.gitignore` file.

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Commit messages:
 - ignore .idea folder
 - Improve error reporting around missing LLVM paths in the gradle build.

Changes: https://git.openjdk.java.net/jextract/pull/8/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=8&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903134
  Stats: 19 lines in 2 files changed: 18 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jextract/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/8/head:pull/8

PR: https://git.openjdk.java.net/jextract/pull/8