RFR: 7903134: Improve error reporting in jextract gradle build [v2]

[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.

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  libclang_home -> llvm_home

-------------

Changes:
  - all: https://git.openjdk.java.net/jextract/pull/8/files
  - new: https://git.openjdk.java.net/jextract/pull/8/files/2237d18b..203f888e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jextract&pr=8&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jextract&pr=8&range=00-01

  Stats: 9 lines in 2 files changed: 0 ins; 0 del; 9 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