Refactor: move error variables from host/static to db (#68)

The error variables defined in host/static are generic and can be reused
by other packages.

internal/db/error.go

@@ -0,0 +1,22 @@
+package db
+
+import "errors"
+
+// Errors returned from this package may be tested against these errors
+// with errors.Is.
+var (
+	// ErrInvalidPublicId indicates an invalid PublicId.
+	ErrInvalidPublicId = errors.New("invalid publicId")
+
+	// ErrInvalidParameter is returned by create and update methods if
+	// an attribute on a struct contains illegal or invalid values.
+	ErrInvalidParameter = errors.New("invalid parameter")
+
+	// ErrNotUnique is returned by create and update methods when a write
+	// to the repository resulted in a unique constraint violation.
+	ErrNotUnique = errors.New("unique constraint violation")
+
+	// ErrNilParameter is returned when a required parameter is nil.
+	ErrNilParameter = errors.New("nil parameter")
+)
+