API change: Column methods should throw exceptions consistently

There are multiple places throughout the library that are really relaxed about input values\types, like:
- overrides of `Column::Append(ColumnRef column)` that just do nothing if column is of not expected type
- overrides of `Column::Load` that return false if column failed to load, sometimes eating an exception that could point to the root cause.
- etc.

Fixes scheduled for next API-breaking release (3.0?):
- `Column::Append` overrides should throw an exception if the input column is of the wrong type.
- `Column::Load` should change return type to `void` and throw an exception on error.