fix: add graceful fallback when ColorBrewer palette loading fails offline

[tom832]

Problem

_load_colorbrewer() calls urllib.request.urlopen(url) without any error handling. When the network is unavailable (offline, DNS failure, firewall, etc.), this raises an unhandled exception that prevents PyMOL from starting normally.

Solution

• Wrap the HTTP request in a try/except block
• Add timeout=5 to avoid long hangs on slow/unreachable networks
• On failure, print a friendly warning and return early — PyMOL continues startup without ColorBrewer palettes
• No other code is changed

Changes

-    js_text = urllib.request.urlopen(url).read().decode("utf-8")
+    try:
+        js_text = urllib.request.urlopen(url, timeout=5).read().decode("utf-8")
+    except Exception as exc:
+        print(f"Warning: unable to load ColorBrewer palettes ({exc}). Skipping.")
+        return