hard-to-understand error with non-existing namers

[wmorgan]

If you put a namer in the dtab that linkerd doesn't know about, you get the following behavior:

• The log says something unintelligible:

E 0201 20:23:21.968 THREAD19: Exception propagated to the root monitor!
Failure(io.l5d.http.anyMethodPfx, flags=0x100000000)
    with Service -> 0.0.0.0/4140
    at com.twitter.finagle.NoStacktrace(Unknown Source)

• The name of the unknown namer is returned as a text string with a 503 error (at least, with HTTP--haven't tried other protocols).

Ideally, the log message would be more informative. For the HTTP error, the 503 part is good, but ideally, the content would be either nothing or a more descriptive error message.