Clarify missing java error message

[rjernst]

Since the bundled jdk was added to Elasticsearch, there are now 2 ways
java can be missing. Either JAVA_HOME is set but does not exist, or the
bundled jdk does not exist. This commit improves the error messages in
those two cases, and also ensures our tests cover both cases.

closes #44139

[elasticmachine]

Pinging @elastic/es-core-infra

[nik9000]

@elasticmachine, run elasticsearch-ci/docs

[pugnascotia]

The changes look good.

I wondered if we should sync the control flow of elasticsearch-env and elasticsearch-env.bat, since the former checks whether JAVA is executable in both the bundled and non-bundled branches, whereas the latter has a check after both branches.

[douglasawh]

UPDATE: figured it out. that user didn't own the entire file path. I'll leave the rest in case others run into the issue.

I still see this error in 7.11.2

./elasticsearch
could not find java in bundled jdk at /root/downloadedSource/elasticsearch-7.11.2/jdk/bin/java

sestatus
SELinux status: enabled
SELinuxfs mount: /sys/fs/selinux
SELinux root directory: /etc/selinux
Loaded policy name: targeted
Current mode: permissive
Mode from config file: enforcing
Policy MLS status: enabled
Policy deny_unknown status: allowed
Max kernel policy version: 31

My user owns those files and directories, so not sure why it couldn't find it