fix: handle no gateway priority configured case

[Mrsandeep27]

Summary

Fixes #13 — when a channel has no gateway priority configured, the handler now logs and skips cleanly instead of raising ZeroDivisionError.

Problem

`PriorityGatewaySelection.select_gateway` computes:
```python
total_gateways = min(len(cls.get_default_priority()), len(current_priority))
return current_priority[n_attempts % total_gateways]
```
If both `default_priority` and `current_priority` are empty (channel configured with no priorities, or dynamic-priority expression evaluated to `[]`), `total_gateways` is `0` and the modulo raises `ZeroDivisionError`.

Additionally, the existing code silently swallows errors from `eval(priority_logic_expression)` — the comment says "Log the exception and fallback to default priority order" but the `except` block is just `pass`.

Fix

1. Introduce `NoGatewayPriorityConfigured` exception. `select_gateway` raises it when both evaluated and default priorities are empty.
2. Log the eval failure (previously silenced).
3. In `AbstractHandler.notify`:
   • Short-circuit if `ENABLED_GATEWAYS_COUNT` is 0.
   • Catch `NoGatewayPriorityConfigured` and log the channel instead of crashing.

Test plan

• Channel with empty `default_priority` and no `dynamic_priority` → logs `No gateway priority configured for channel `, no crash, `notify` returns `(None, None)`.
• Channel with valid `default_priority` → behaves exactly as before.
• Channel with malformed `dynamic_priority` → logs the eval failure, falls back to `default_priority`.
• Channel with `ENABLED_GATEWAYS_COUNT` of 0 → early return with log, no call to `select_gateway`.

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