color: ignore effects missing from terminfo

If terminfo mode is in effect, and an effect is used which is missing from
the terminfo database, simply silently ignore the request, leaving the
output unaffected rather than causing a crash.

hgext/color.py

@@ -378,7 +378,10 b' def _effect_str(effect):'
     if effect.endswith('_background'):
         bg = True
         effect = effect[:-11]
+    try:
     attr, val, termcode = _terminfo_params[effect]
+    except KeyError:
+        return ''
     if attr:
         if termcode:
             return termcode