Print a better error when failing to load middleware

[icambron]

Finally getting around to addressing #34. Demo:

-> lein trampoline repl
REPL-y 0.3.5, nREPL 0.2.10
Clojure 1.6.0
Java HotSpot(TM) 64-Bit Server VM 1.8.0_45-b14
    Docs: (doc function-name-here)
          (find-doc "part-of-name-here")
  Source: (source function-name-here)
 Javadoc: (javadoc java-object-or-class-here)
    Exit: Control+D or (exit) or (quit)
 Results: Stored in vars *1, *2, *3, an exception in *e

zs.client.browser-repl=> (piggieback/cljs-repl (weasel/repl-env :ip "0.0.0.0" :port 9003))
Can't set root bindings. This is likely caused by the nrepl middleware failing to load. You may have misconfigured it, or it could be a result of running in an unsupported environment (specifically `lein trampoline repl` generally doesn't work right; run without trampoline). Exact exception below.

IllegalStateException Can't change/establish root binding of: *cljs-ns* with set  clojure.lang.Var.set (Var.java:221)