ShadowSpawn should fail right away when the source path does not exist

[candera]

Motivation

Currently, if you specify a non-existent source path, ShadowSpawn will fail, but not until it has made the shadow copy. If we fail right away, the user won’t have to wait for the shadow copy to get created.

Acceptance Criteria

• Return a specific error prior to creating a shadow copy when the source directory does not exist.