Add IF NOT EXISTS to alter blueprint

[dextermb]

This can be accomplished by using the PRAGMA functions.

P.S: The alter blueprint is also called the "Alert" blueprint

[jaredkove]

Would we want this? would this result in the silent failing of migrations if anything were to be present when it shouldn't have been?

[dextermb]

Would we want this? would this result in the silent failing of migrations if anything were to be present when it shouldn't have been?

Currently how one of our projects is structure, it will error if you clear the database. This is because the migrations are attempted to be ran while the internal DB handler also tries to add columns.

Users should never see this issue in production, but it would make development smoother.

[dextermb]

This actually may be more of an issue then first anticipated as any new devices will start with a fresh DB, causing the collision.

cc: @robmilward