Better error handling

hdinterval.py

@@ -65,6 +65,8 @@ class HDInterval:
         return self._at(target, tolerance, ci, jump)
 
     def at(self, target, tolerance=1e-3):
+        assert tolerance > 0
+
         while tolerance < 1:
             try:
                 return self._at(target, tolerance, 1, 1)
@@ -72,4 +74,4 @@ class HDInterval:
                 tolerance *= 10
                 warnings.warn(f'Tolerance reduced: {tolerance}')
 
-        raise OverflowError()
+        raise OverflowError('Unable to converge')