Bug: fetch.py swallows ALL exceptions silently — impossible to debug OSM fetch failures

## Bug: `fetch.py` swallows ALL exceptions silently — impossible to debug OSM fetch failures

### Describe the bug

`fetch.py` has two bare `except Exception as e:` blocks that catch and discard every error. When Overpass returns a 429, a timeout, a malformed JSON, or a partial payload, the error is completely swallowed. The user sees `Fetching geodataframes took 120s` followed by a downstream `KeyError` with no indication that the actual problem was an Overpass rate-limit.

### To reproduce

```python
import prettymaps
prettymaps.plot("Zurich, Switzerland", radius=1800)
# → Fetching geodataframes took 128.19 seconds
# → KeyError: 'highway'   ← no indication that Overpass returned empty/partial data
```

### Expected behavior

Errors during OSM data fetching should be logged or surfaced so the user can understand why a render failed and take action (retry, use a different Overpass endpoint, reduce radius, etc.).

### Root cause

**Line 609** — `unified_osm_request` main fetch:
```python
try:
    all_features = ox.features.features_from_polygon(bbox, tags=combined_tags)
except Exception as e:
    all_features = GeoDataFrame(geometry=[])   # silently empty, no log
```

**Line 692** — per-layer split:
```python
except Exception as e:
    # print(f"Error fetching {layer}: {e}")    ← commented out!
    gdfs[layer] = GeoDataFrame(geometry=[])
```

### Suggested fix

1. Uncomment the print/log line, or use the `logging` module:
```python
except Exception as e:
    logging.warning(f"Error fetching layer '{layer}': {e}")
```
2. Distinguish recoverable errors (empty result) from fatal ones (auth, network):
```python
except (ConnectionError, TimeoutError) as e:
    raise RuntimeError(f"Overpass API unreachable: {e}") from e
except Exception as e:
    logging.warning(f"Layer '{layer}' fetch failed: {e}")
```
3. Surface Overpass 429 responses with a retry hint and `User-Agent` header guidance.

### Environment

- prettymaps v1.4.2
- Python 3.10.18 (Homebrew)
- osmnx 1.2.2
- macOS 15, ARM64


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Bug: fetch.py swallows ALL exceptions silently — impossible to debug OSM fetch failures #159