[code-reflection] Integrated: Add env.bash error tracking and update build scripts and docs

[Gary Frost]

Added some error checking to `env.bash`

Specifically ensure `env.bash` is sourced `. ./env.bash` rather than executed by the shell.

Allow for `hat` dir to be seperated from `babylon jdk`

`env.bash` now also builds minimal `bldr/bldr.jar`

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Commit messages:
 - Add env.bash error tracking and update build scripts and docs

Changes: https://git.openjdk.org/babylon/pull/261/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=261&range=00
  Stats: 492 lines in 10 files changed: 81 ins; 382 del; 29 mod
  Patch: https://git.openjdk.org/babylon/pull/261.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/261/head:pull/261

PR: https://git.openjdk.org/babylon/pull/261