acc: improve diagnostics for algorithms that use too many colors

* spot/twa/acc.hh (acc_cond::mark_t): Diagnose mark_t with set numbers
that are too larges.
* tests/python/except.py: Adjust.
* tests/core/acc.cc: Remove most of asserts, as those can be disabled,
and adjust expected exception.

tests/python/except.py

@@ -169,7 +169,7 @@ else:
 try:
     m = spot.mark_t([0, n, 1])
 except RuntimeError as e:
-    assert "bit index is out of bounds" in str(e)
+    assert "Too many acceptance sets used.  The limit is" in str(e)
 else:
     report_missing_exception()