Use proper exception types in task

[f4lco]

In reference to PR https://github.com/ReviewNB/treon/pull/9#discussion_r298601878 this fixes following shortcomings:

• Linting violation for overly broad exception
• Additional whitespace before starting the traceback
• Showing treon's traceback despite that only the traceback within the cell is relevant

[amit1rrr]

Looks good, merged! Thanks @f4lco