Error messages from SQL plugin

[joesiltberg]

At least the sql_plugin_next_row_func function should be able to report an error message when something goes wrong.

In the ODBC plugin we only return an error in one place, at that point we also have an exception so we should ask its what() function for more information and return it (like how the init function returns an error).

Also perhaps sql_plugin_header_func.

Note that this will potentially break backwards compatibility, should we introduce a sql_plugin_next_row_ex_func? Perhaps not necessary since we typically release client and plugin together...