Throw exceptions for invalid dependency rows.

Makes MySQL behave more like the other engines. Enabled by the introduction
of the `SIGNAL` syntax introduced in MySQL 5.5.
Add a simple procedure to throw an exception.

And use it in the triggers. It can also be used in verify scripts, assuming
that the sqitch schema is known at runtime.
Replace raise() with checkit().

The latter is defined as both a procedure and a function. We use the procedure
to mimic a CHECK constraint in the triggers on the `dependencies` table. The
function we don't use directly, but provide to make it easier to write verify
tests.