improve error handling, through additional exception handler in done_callback

[maurerle]

These exception fail silently otherwise.

We can think about letting users handle this on their own by having a construct like, though I like to provide users with a decent error messages by default:

def handle_exception(loop, context):
    # context["message"] will always be there; but context["exception"] may not
    msg = context.get("exception", context["message"])
    logger.exception(f"Caught exception: {msg}")

...

self.loop.set_exception_handler(handle_exception)

and adding this to the task loop through set_exception_handler

We also reduce the log messages in the distributed_clock as this is more of an info

[codecov]

Codecov Report

Attention: Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.85%. Comparing base (f89fd83) to head (be6d444).
Report is 2 commits behind head on development.

Files with missing lines	Patch %	Lines
mango/util/scheduling.py	60.00%	4 Missing ⚠️
mango/util/distributed_clock.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #144      +/-   ##
===============================================
- Coverage        88.97%   88.85%   -0.12%     
===============================================
  Files               22       22              
  Lines             2458     2468      +10     
===============================================
+ Hits              2187     2193       +6     
- Misses             271      275       +4     

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