improve exception when contract cannot be loaded

src/clj/commiteth/eth/token_registry.clj

@@ -37,6 +37,7 @@
   ([addr]
    (println "addr" addr)
    (let [contract (load-tokenreg-contract addr)]
+     (assert contract (format "Could not load contract for addr %s" addr))
      ;(assert (.isValid contract)) ;; web3j's isValid can't be trusted...
      (let [token-count (-> contract .tokenCount .get .getValue)]
        (log/debug "token-count" token-count)