use column_database_name etc accessor functions for better error messages

[nurpax]

When hitting conversion errors in type conversion in row/field parsers, the function in:

http://www.sqlite.org/c3ref/column_database_name.html

might be useful for more descriptive error message - it'd be helpful to show the name of the offending table and column that was erroneous.

[nurpax]

Hmm, it'd require a bit of plumbing to pass down the current Statement down to the FromField parsers. It's a bit messy now, probably should refactor how this works to make it easier to improve error message construction. Perhaps close out issue #23 while at it.