Add documentation about plugin enabling on pex

[rtibbles]

Summary

Adds context aware error messaging to plugin enable and apply to give information about pex plugin enabling Adds developer documentation about the same.

References

Fixes #6516

Reviewer guidance

Try to enable a non-existent plugin with the pex file - see the error message!

:robot: This was created by Claude Code. @rtibbles then reviewed the generated output, and did iterative rounds of updates before making it ready for review :robot:

[github-actions[bot]]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.19.0b2.dev0_git.98.ga0b51a44.pex
Windows Installer (EXE)	kolibri-0.19.0b2.dev0+git.98.ga0b51a44-windows-setup-unsigned.exe
Debian Package	kolibri_0.19.0b2.dev0+git.98.ga0b51a44-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.19.0b2.dev0+git.98.ga0b51a44.dmg
Android Package (APK)	kolibri-0.19.0b2.dev0+git.98.ga0b51a44-0.1.6-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.19.0b2.dev0+git.98.ga0b51a44.zip
TAR file	kolibri-0.19.0b2.dev0+git.98.ga0b51a44.tar.gz
WHL file	kolibri-0.19.0b2.dev0+git.98.ga0b51a44-py2.py3-none-any.whl

[rtibbles]

So much for trusting the issue!

[rtibbles]

Looks like fallback is probably the best value.

[rtibbles]

Although.. weirdly that doesn't seem to work.

[rtibbles]

Oh, no, it does! Nevermind - updating!

[rtibbles]

Updated to PEX_INHERIT_PATH=fallback, which does seem to work - I guess the 1 value was from pex v1, which we dropped when we dropped Python 2.7 support.

[rtibbles]

Verified here for the version of pex we are using, as a reference: https://github.com/pex-tool/pex/blob/v2.1.153/pex/variables.py#L487

[bjester]

Yeah fallback seems most appropriate. I will test it to double check