Make migration logic deprecated

This commit deprecates the migration logic from the
sequelize core references the rewritten CLI and the
new migration engine. Also this commit removes the
respective migration tests.
add `.original` property to custom Error classes

The current `.parent` property is a bit of a misnomer. `.original`
better conveys the intent of the field and function of the class, which
is to wrap the original database error.

This commit does not remove the current `.parent` property for backwards
compatibility.
Merge pull request #2761 from defunctzombie/original-error

add `.original` property to custom Error classes
Merge pull request #2750 from sequelize/hotfix/remove-migrator

Mark migration logic as deprecated.
remove UniqueConstraintError in favor of ValidationError

Unique constraint errors are in fact a different type of validation
error, run on the database level. Creating a specialized error for
this case adds needless complexity for users when a ValidationError
already occurs. This patch aims to replace UniqueConstraintError
completely with a ValidationError.