Only attempt blockage migration if reachability credential is missing
I noticed a failure in lox-wasm integration tests for the check_blockage and blockage_migration protocols. From what I can tell, these tests will only work if the bridges that belong to the credential are marked as blocked in the lox distributor. If no bridges have been blocked, the user will not have an entry in the migration table and the blockage migration credential decryption will fail.
We'll have to update the integration tests, but it made me realize that the Tor Browser integration should really be pulling the most recent encrypted bridge table and checking for any missing reachability credentials first before attempting a blockage migration. We should also add some more accessible client-side functions (or errors) to catch whether a blockage migration should be attempted rather than going through all the steps only to have the table decryption fail and give us an opaque "Verification failed" error.