CA-112324: Check destination SR's connectivity during VM migration.

Issue: During VM migration if the destination SR is not attached , then the migration fails with a bad error message with an ungraceful view

Fix: Defined an error message in api_errors.ml namely "sr_not_attached". Catching the exception in storage_migrate.ml before mirroring starts and displaying it in a graceful manner.

Signed-off-by: Ravi Pandey <ravi.pandey@citrix.com>