Don't use `assert` for control flow

`assert` keyword can be disabled by interpreter, so it's not reliable
for functional code.

sznqalibs/hoover.py

             op, items = pattern
         except TypeError:
             raise ValueError("pattern is not a tuple: %r" % pattern)
-        try:
-            assert issubclass(op, _BaseRuleOp)
-        except TypeError:
+        if type(op) is not type:
             raise ValueError("invalid operator: %r" % op)
-        except AssertionError:
+        if not issubclass(op, _BaseRuleOp):
             raise ValueError("invalid operator class: %s" % op.__name__)
         return bool(op(items, item_ok))
 
 
         def _validate(self):
             try:
-                assert self._path.startswith(self.DIV)
-            except (AttributeError, AssertionError):
-                raise ValueError("invalid path: %r" % self._path)
+                has_root = self._path.startswith(self.DIV)
+            except AttributeError:
+                raise ValueError("invalid path: not a string: %r" % self._path)
+            if not has_root:
+                raise ValueError("invalid path: missing root: %r" % self._path)
 
         def stripped(self):
             return self._path.lstrip(self.DIV)