API change: Column methods should throw exceptions consistently

[Enmk]

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.

[1261385937]

Just like lib boost, error_code and exceptions are both supported if API-breaking is sure