API: to_datetime, required unit with numerical (pandas-dev#15836)

* Tests
  * add test_to_datetime_numerical_input
  * add fixtures
  * update bool tests
* Check argument
  * for numerical type
  * DataFrame with unit

Author: atbd

pandas/_libs/tslib.pyx

@@ -2294,7 +2294,7 @@ cpdef array_with_unit_to_datetime(ndarray values, unit, errors='coerce'):
             else:
 
                 if is_raise:
-                    raise ValueError("non convertible value {0}"
+                    raise ValueError("non convertible value {0} "
                                      "with the unit '{1}'".format(
                                          val,
                                          unit))