sql.py: Added table_exists() and errors_table()

sql.py: DbConn.print_notices(): Fixed bug where it should not do anything for a MySQL connection, because that doesn't store notices the way Postgres does

sql.py: put_table(): MissingCastException: Debug message: Added Redmine formatting

schemas/functions.sql, vegbien.sql: Removed no longer needed cast functions, which are now created on the fly by column-based import

schemas/functions.sql: _nullIf(): Ignore uncastable value, because a value that's invalid for the given type is still well-defined as not matching the nullif() criterion

sql.py: put_table(): MissingCastException: Debug message: Removed "'s" so it wouldn't mess up syntax highlighting when pasting debug output into a SQL file

sql.py: cast(): Made errors table also store SQLSTATE in error_code column

sql.py: cast(): Documented that the value and error are inserted for each source column (hence the CROSS JOIN)

sql.py: cast(): Version the function name if using an errors table, to avoid collisions with other cast functions when the function name is truncated (or, more rarely, collisions with casts to the same type and on the same input columns but of a different table)

sql.py: cast(): function_name: Fixed bug where sql_gen.FunctionCall()'s parameters needed to be passed with *args syntax