ProcessMonitorWidget: Print traceback for callback exceptions

[danielhollas]

Closes #386 (see the issue for details why this change is needed)

Before:

After:

NOTES:

1. The warning is hidden in the Appmode. This can easily lead to user confusion, since something doesn't work but the error is hidden. Not sure if this is the best design, perhaps these exceptions should be raised instead. But that's beyond the point of this PR.
2. This fix could (should?) be applied to the 1.x support branch.

[danielhollas]

@yakutovicha here's a code snippet to test this in Jupyter Notebook

from aiida.orm import load_node
from aiidalab_widgets_base import ProcessMonitor

def raise_error():
    raise ValueError("We have a problem")

monitor = ProcessMonitor(
            timeout=0.5,
            callbacks=[
                raise_error,
            ],
)
process_pk = 1217
monitor.value = load_node(process_pk).uuid

[yakutovicha]

Thanks, @danielhollas. I just have a minor request.

[danielhollas]

@yakutovicha thanks, I've accepted your suggestion.

Once this is merged, could you please release a new beta version? I want to use this for better in my app for easier debugging.
Also the last beta version is broken for my app so it would be nice to publish the configure_tabs fix.

[yakutovicha]

@yakutovicha thanks, I've accepted your suggestion.

Once this is merged, could you please release a new beta version?

sure, will do that right away