When a block registered with `Database#define_function` raised an
exception, the connection was left holding sqlite's per-connection
mutex. Any other thread that subsequently used the connection blocked
forever while holding the GVL, wedging the entire process.

The scalar function callback will now invoke the block under
`rb_protect`, following the pattern established by the aggregate
handlers in `aggregator.c`: a raise will be converted into
`sqlite3_result_error` so that sqlite unwinds and releases its mutex
normally, and `Statement#step` will re-raise the original exception to
the caller.