Raise exceptions from environment subprocesses

[harperj]

Proposed change(s)

This commit surfaces exceptions from environment worker subprocesses,
and changes the SubprocessEnvManager to raise those exceptions when
caught. Additionally TrainerController was changed to treat environment
exceptions differently than KeyboardInterrupts. We now raise the
environment exceptions after exporting the model, so that ML-Agents will
correctly exit with a non-zero return code.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[ervteng]

Looks good to me - should we add a test that makes sure the exception is passed correctly up?

[harperj]

@ervteng Good timing :-)

Just pushed a test showing we raise the exception from SubprocessEnvManager.

[ervteng]

LGTM!