ENH: Capture warnings.

[tobiasraabe]

Is your feature request related to a problem?

Warnings are not separately captured from stdout or stderr and will not be displayed when a task succeeds.

Describe the solution you'd like

Warnings should be displayed at the end of a session so users can improve their code.

Also document how to turn warning into exceptions, PYTHONWARNINGS=error, and use the debugger to get rid of them.

API breaking implications

None

Describe alternatives you've considered

• Should warnings which are ignored by default.