Replace bare except clauses with specific exception handling

[Copilot]

Addresses code review feedback on PR #116 regarding bare except: clauses that silently catch all exceptions without documentation.

Changes

• Line 1123: Replace except: with except TradeRequest.DoesNotExist: when attempting to retrieve timetable_slug from a trade request
• Line 1159: Replace except: with except TradeMatch.DoesNotExist: when attempting to retrieve timetable_slug from a trade match
• Add explanatory comments documenting the intended fallback behavior when these lookups fail

Before:

try:
    trade_request = TradeRequest.objects.get(pk=request_id)
    timetable_slug = trade_request.offered_allocation.timetable.slug
except:
    pass

After:

try:
    trade_request = TradeRequest.objects.get(pk=request_id)
    timetable_slug = trade_request.offered_allocation.timetable.slug
except TradeRequest.DoesNotExist:
    # If trade request doesn't exist, timetable_slug remains None and default view is shown
    pass

This prevents accidentally catching system exits, keyboard interrupts, and unrelated exceptions while making the code's intent explicit.

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